Fruit Tree Diseases
Host: Wild and cultivated apple and crabapple.
Leaves -- The first infections often occur on the leaves surrounding the flower buds. The appearance
of dull, smoky areas on the leaf, 8 to 18 days after infection, is the first evidence of the
disease. These areas soon become olive-colored and velvety. As the lesions (infected areas)
become older, they assume a definite outline as olive-green or nearly black circular spots. As
other leaves develop, they are equally susceptible. Severe early leaf infection can result in
dwarfed, twisted leaves, which later drop.
Fruit -- Early infection may occur on the calyx (blossom end of the fruit) or on the pedicel (fruit
stem). Severe pedicel infection results in fruit drop. Fruit may become infected at any time in
their development. Typical fruit lesions are distinct, almost circular, rough-surfaced, olive-green
spots up to 3/4 inch in diameter. Heavily infected fruits are usually misshapen and they may
crack and drop prematurely. When leaf infection is active just before harvest, the fruit may
become infected. These spots do not show at harvest time, but they will develop slowly while the
apples are in storage. This phase of the apple scab disease is termed storage scab. Infected fruit,
when removed from storage, will show numerous, small, black spots. These spots are sometimes
confused with bitter pit or ammonia injury.
Disease Cycle -- The apple scab fungus (Venturia inaequalis) overwinters in infected leaves that
have fallen to the ground. Fruiting bodies are produced within the dead leaf tissue. As spring
approaches these begin to mature and produce spores (ascospores) that are discharged into the air
currents and carried to the developing apple buds. The fruiting bodies in the fallen leaves must
be wet for spore discharge to take place. All the ascospores are not discharged with the first
spring rains for they mature over a 4 to 6 week period. This usually coincides with the period
from when apple buds show 1/4 inch green until 2 to 3 weeks after petal fall.
When these spores land on wet apple buds, leaves, or fruit and if they remain wet for a few
hours, the spores will germinate and send sprouts (germ tubes) into the apple tissue. The time
required for germination and penetration depends on temperature and the presence of a wet
surface. At 40o F almost 48 hours of continuous wetting is required for infection, while at 65-75o
F, only 10 hours are required. After the fungus has penetrated, it continues to grow and enlarge
beneath the cuticle. After 8 to 18 days (development occurs most rapidly at high temperatures) a
visible scab lesion is produced. On the surface of the lesion more spores (conidia) are produced.
These spores are easily dislodged when the lesions are wet. They are splashed around by rain
and blown by wind to new leaf and fruit surfaces within the tree. These spores germinate on wet
surfaces, infect the tissue, and produce a new lesion. In this manner, several secondary infection
cycles may occur in the course of a growing season.
Chemical Control -- Scab infections may be prevented by applications of fungicides at regular
intervals throughout the growing season. The object is to provide a protective coating that will
inactivate any spores landing on the fruit and foliage. Spray applications are more effective than
The critical period for scab control is early in the season from bud emergence through second
cover period. If scab infection can be prevented during the time all the ascospores are discharged
from the fruiting bodies in the fallen leaves, the disease cycle is broken and no further source of
infection remains for the rest of the season. However, if control is not obtained, and leaf and fruit
infection does occur, then conidia are produced on the lesions and scab will remain a constant
threat all season whenever wet weather occurs.
Choice of Fungicides -- Scab control fungicides may be protectants or dual action materials that
combine both protectant and eradicant properties. The protectant-type materials must be present
on the leaf or fruit surface before the spores land in order to be effective. Materials of this type
are Ferbam, Glyodin, Thiram and wettable sulfur. If the germ tube has already penetrated the
leaf or fruit these materials will not stop infection.
In contrast, an eradicant fungicide can be applied to the infected surfaces after the infection
has occurred and stop the infection process. This property of certain fungicides is quite useful as
the grower may apply the fungicide a short time after the beginning of a rain period and still
Some fungicides have combined eradicant-protectant properties. Examples of these materials
are Benomyl, Captan, Dodine, Dikar, Glyodex, and Polyram. Dodine and benomyl, when
sprayed on existing scab lesions, will reduce spore production in the lesion, a property of great
help to the fruit grower.
Hosts: Cherry, peach, nectarine, plum, apricot, apple and pear.
While this disease can occur on all stone fruit trees, and on apple and pear blossoms, it is important
only on sweet cherries and ornamental flowering cherries in the Northeastern U.S. It is
caused by the bacterium Pseudomonas syringae. Several other names have been used for the
same disease; the more common are gummosis and sour sap.
Gumming is common on stone fruit trees, whether on trunks, limbs, twigs, or fruit when
injuries and fruit malformations occur. Thus, the name gummosis does not define a cause--only
a response. Sour sap often occurs following winter injury to the trunks or limbs, wet feet, and
soil-line girdling in the winter or early spring. The sour odor is due primarily to the fermentation
of sugars by yeast under the bark of injured areas. So this term, too, does not describe a cause.
Even so, gumming and sour sap conditions are symptoms of the bacterial canker disease.
Symptoms on sweet cherry -- The disease occurs on branches, twigs, buds, leaves, and fruit. The
most conspicuous symptoms are the cankers and dying branches girdled by them. The cankers
on the twigs are darkened areas often at the base of buds. On the limbs or trunks they often are
darker than the normal bark, sunken in their centers, and they may extend for a considerable
distance. Gumming is frequent in the spring and fall of the year when the disease is most active.
Leaves and shoot growth beyond the canker may wilt and die during the growing season when
the cankers girdle a branch or the trunk. Leaf and flower buds are killed during the dormant
season, probably as a result of infection during the fall. Small cankers often develop at the base
of these dead buds. At times, infected fruiting spurs blossom normally only to wilt and die
During periods of cool, wet weather after bloom, leaf and fruit infections may be common.
The leaf spots are mostly angular in shape and dark purple, brown, or black in color. The infected
areas may drop out leaving a tattered appearance to the leaf, or the entire leaf may yellow
and fall. Fruit infection shows as deep, black depressions, as do infections on the fruit stems.
Disease Cycle -- The causal bacteria overwinter in the margins of cankers in the wood and in
infected buds. In the spring, during wet periods, the bacteria multiply and ooze from the cankers.
They are spread by rains and enter the plant through natural openings or wounds. Periods of
frequent rains, cool temperatures, and high winds are most favorable for infection. Frost-injured
leaves and blossom spurs and cold-injured trees seem especially susceptible. With the higher
temperatures of late spring and summer, disease development stops. At this time the newly
formed buds become infected either through the leaf scars or bud scales, or both.
Control -- While bacterial canker can be a severe disease in itself, it is often much more severe on
cold-injured trees and on trees growing in sites with poor internal soil drainage.
The causal bacteria can be transmitted by pruning tools so these should be disinfected between
trees if bacterial canker is present. Limbs with cankers on light to moderately infected
trees should be cut off several inches below the cankered area. This can be done during a warm,
dry period of summer, or during the winter when temperatures are freezing or below.
Sprays during the growing season have not been effective in disease control. Some benefit
has come from applications made when most of the leaves have dropped in the fall and just
before bud swell in the spring. For maximum benefit, these sprays should be continued for
several years on susceptible trees.
Hosts: Peach, nectarine, plum, apricot and cherry.
This bacterial disease, caused by Xanthomonas pruni, affects peach, nectarine, plum, and apricot
most seriously. Other names for the disease are bacteriosis, shot hole, and spot. The causal
bacteria attack fruit, leaves, and twigs. Fruit loss on some cultivars can be very high in occasional
years. Early and severe defoliation can affect fruit size and winter hardiness of buds and
wood. Bacterial spot is more common and severe when trees are growing in light, sandy soils
and when nitrogen is insufficient for normal tree growth.
Symptoms -- The symptoms of bacterial spot are quite different from other diseases of stone
fruits. They may be confused, however, with injuries from some spray materials and nitrogen
On the leaves, the disease is noticed first as small, angular spots that are purple, black, or
brown in color. The mature spots remain angular and about 1/3 to 1/4 inch in diameter. Often
the spots are most numerous at the tip ends and along the midribs of the leaves. While the
individual spots are small, several may join thus involving a large part of the leaf. The infected
areas may drop out giving the infected leaves a shot hole appearance. Infected leaves eventually
turn yellow and drop. There is no apparent relationship between the number of individual spots
and the yellowing and defoliation. On plum, the shot hole effect is more pronounced than on the
other stone fruits.
Other leafspot diseases and spray injury spots tend to be much more circular in outline.
Often, these are not confined by the veins in the leaf as is bacterial spot. Nitrogen deficiency
symptom spots are normally red in color.
On peach, nectarine, and apricot fruit, bacterial spot is first noticed as small red or brown
surface spots. They may appear anywhere on the fruit, but similar-age infections often are
concentrated in the same area. Young spots frequently have a red, green, or light yellow halo
around them. As the fruit grows, the spots become black and depressed. Since the infected areas
cannot expand with increased fruit size, the spots crack. Fruit infected early in its development
will have deep, ugly pits. When the spots are numerous, several deep cracks will develop in the
affected area of the fruit.
On plum the fruit symptoms are likely to be quite different. Here, large, black, sunken areas
are most common. On a few cultivars, small pit-like spots occur.
There are two distinct twig cankers on peach and nectarine. Summer cankers develop on
green shoots in mid- to late-summer. These are small to quite large purple-black lesions, slightly
sunken to deeply cracked, and circular to elliptical in shape. Spring cankers develop on the
previous seasonís growth beginning from about the time of bud swell through the bloom period.
They may appear as small, somewhat blister-like, darkened areas often around or near a bud.
Later, the epidermis ruptures and the bacteria become exposed. Spring cankers also are seen as a
tip dieback of the twig and the bacteria become exposed at the junction of diseased and healthy
The cankers on plums and apricots eventually appear quite different. In the susceptible
cultivars of these fruits, the bacteria may survive for 2 or 3 years slowly enlarging and deepening
the cankered area. The results are deep-seated cankers deforming the small branches so they
have a knotty appearance. Some of these branches may be killed or they may break from the
weight of the fruit.
Sweet and tart cherry leaves may be affected by the bacterial spot disease. While rarely
happening, the leaf symptoms are like those on peach.
Disease Cycle -- The bacteria overwinter in the twigs or buds. These infections take place in the
late summer or fall of the preceding season in peach and nectarine, and result in the spring canker
or tip dieback. The time when the bacteria enter the buds or twigs, and under what environmental
conditions this occurs, has not been discovered. In plum and apricot the organism does live
over in summer cankers, often for at least two seasons.
First infections in the spring can occur anytime after the leaves begin to unfold. Temperatures
above 65o F and warm rains are needed for the bacteria which are rarely noticed but do
initiate the disease each year. The severity of the secondary infections depends entirely on the
weather. A moderately warm season with light, frequent rains accompanied by heavy winds
favor severe outbreaks of bacterial spot. Any recent injury to the leaves or fruit, such as windblown
soil particles and hail, may result in severe outbreaks. Twig infections resulting in summer
cankers occur in mid- to late summer. The twig, bud, and possibly leaf scar infections that
result in spring cankers and tip dieback occur in late summer and fall.
Control -- Maximum use of resistant cultivars is the most effective control measure. There are
growing numbers of good peaches highly tolerant of the bacterial spot disease. The leaflet
"Peach Varieties for the Seventies," rates many of them as to their susceptibility. Resistance in
plums, nectarines, and apricots is not as common. Nurserymen are well aware of the degree of
susceptibility of the cultivars they sell and they can provide good information for specific areas.
Since trees in poor vigor are more susceptible, orchard management programs should be designed
to maintain good vigor. Sanitary measures are of little value.
There are no completely successful spray programs for control of bacterial spot. In seasons
when the disease incidence is light, special programs do help. In those when infections are
numerous they still reduce the number of infections, but not enough to prevent defoliation and
many infected fruit.
Hosts: Apple and pear. On peach and nectarine the same fungus causes a disease known as
anthracnose; on grape it causes a disease known as ripe rot; and on chestnut a disease known as
blossom-end rot of green burs. The discussion below is limited to apple and pear.
Symptoms -- The disease will be seen only on the fruit. Cankers can form on twigs, but they are
rare. The fungus does not infect leaves; however, it is one of the few fruit rot organisms that can
penetrate the unbroken skin of the fruit. The disease is noticed first, during midsummer or later,
as a small, light-brown, circular spot. One or many may appear on a fruit. They enlarge quite
rapidly if the temperatures are high and soon change to a dark brown color. By the time the spots
are 1/8 to 1/4 inch in diameter they are distinctly sunken or saucer-shaped. When they reach 1/2
inch in diameter, small black dots, the fruiting bodies of the fungus, appear in the sunken lesion.
These may be arranged in concentric rings. Later, they ooze a gelatinous salmon-pink mass of
spores, which are washed by rains to other fruit. Beneath the surface of the spot, the flesh is light
brown and watery in cone-shaped areas with the small end of the cone toward the center of the
fruit. As the fruit ripens, it decays rapidly and finally shrivels into a mummy.
Disease Cycle -- The fungus lives over winter in mummied fruit, and in cracks and crevices in the
bark. The jagged ends of broken limbs are an ideal site. With the advent of warm weather the
fungus produces spores that are washed by rains to the developing fruit. Often the first infections
appear in a cone-shaped area on the tree and can be traced to a source of spores at the tip of the
cone. The optimum conditions for disease development are rains, relative humidity of 80 to 100
percent, and 85o F temperature.
Hosts: Plum and cherry.
This disease is well-named since the black, warty knots on branches of infected trees are its most
characteristic symptom. Trees with the disease, caused by the fungus Dibotryon morbosum,
gradually become stunted and unthrifty. Occasionally, limbs may be girdled. This disease is
most important on plum and secondarily on cherry.
Symptoms -- The disease is present only in the woody parts of trees, occurring most frequently
on twigs and branches and sometimes on trunks and scaffold limbs. The warty swellings first
become visible in late summer or following spring on new shoots. At first the knots are somewhat
greenish and corky. With age they become black and hard. The knots vary in length from
an inch to nearly a foot. Many times they do not completely circle the branch. Those a year or
older may become covered with a pinkish-white mold of another fungus as well as riddled with
insects, especially the lesser peach borer.
Disease Cycle -- About the time the new seasonís growth is one-half inch long, spores of the
fungus are discharged from tiny sacs in the surface of the knots. These are spread by rain and
winds to the new growth where infection takes place. Spore discharge and infection are greatest
during wet periods. Temperatures range from 55o to 75o F. Infections continue to occur until
terminal growth stops. A few greenish, corky swellings may become visible the fall after infection
occurs, but most will not be noticed until the following spring. Generally, the knots produce
no spores until the second spring after they become visible. The fungus in the woody tissues
continues to grow in the spring and fall, increasing the length of the knots. Their eventual size
depends greatly on the species and cultivar of the host.
Control -- New plantings of plums should not be made next to old ones with black knot. Wild
plums and cherries should be removed from nearby woods and fence rows for at least 500 feet
from the new orchard. Once the disease appears in the trees, the knots should be removed.
When they occur on twigs and small branches, prune out the infected branches about 4 inches
below the knot. The knots on large branches and trunks can be cut out. This is done most successfully
during the summer when the fungus does not extend far beyond the visible swelling.
Remove the diseased wood and about one inch of clean wood around the knot. Coat the area
with a tree wound dressing. The knots should be removed before growth begins in the spring,
and taken from the orchard, as they will continue to produce spores for several weeks even after
removal. Once the knots have been removed, sprays will control the disease.
Hosts: Apple, crabapple, pear and quince.
The black rot fungus, Physalospora obtusa, causes a leafspot, fruit rot, and limb canker on apple,
and a fruit rot on pear and quince. The fungus will grow and produce its spores on dead woody
tissue of many plants, but its parasitic activities are confined mostly to the pome fruits. Losses
from black rot result from rotted fruit, defoliation, and loss of limbs by cankers. Fruit rot and
leafspot are more destructive in the southern and central fruit growing areas of eastern United
States, while the limb canker phase is more destructive in the northern areas.
Symptoms -- Fruit rot usually appears at the calyx end of the fruit. It can originate at any wound
that penetrates the epidermis, including those of insect injuries. Ordinarily there is but one spot
per fruit, a difference from bitter rot. When first seen, the spots are brown and this color may not
change as the area increases in size. Or, the original area may turn black. As the spots enlarge,
often a series of concentric bands is formed, the darker bands of mahogany brown to black
alternating with brown bands. As it becomes mature, the entire fruit is rotted, turns black, and
shrivels into a mummy. The rotted area remains level with the healthy flesh for some time,
further distinguishing black rot from bitter rot. The brown flesh of the decayed area is quite firm
and somewhat leathery, especially in immature apples. Sooner or later, tiny, black, pimple-like
bodies (pycnidia) appear on the surface of the rot. These contain spores of the black rot fungus.
The first evidence of the disease on the leaf, also known as frogeye, is the appearance of tiny
purple specks on the upper surface. These enlarge initially to 1/8 to 1/4 inch diameter circles.
The margin remains purple, while the center turns brown, tan, or yellowish brown. Often there is
a second enlargement of leafspots. This is not a uniform expansion, so the spots become irregular
or lobed in shape. By this time the leafspots assume their characteristic "frogeye" appearance:
a purple margin with a zone of dark brown surrounding the tan to gray center. Small,
black pycnidia may appear in the centers of the spots on the upper leaf surface.
The infected areas of branches and limbs are reddish-brown in color and sunken slightly
below the level of the surrounding healthy bark. The cankers may expand a little each year; a
few of them eventually becoming several feet in length. The margins of these older cankers are
slightly raised and lobed. The bark in the center of older cankers usually turns light-colored,
loosens, and scales off raggedly. This characteristic is not confined to black rot cankers so it is
not a good diagnostic symptom. The pimple-like fruiting bodies of the fungus may appear in the
bark of cankered areas. However, they are more frequent and much more numerous in bark of
small dead twigs.
Disease Cycle -- The fungus overwinters in tiny, black, pimple-like bodies (pycnidia) in dead
bark, dead twigs, and mummied fruit. The fungus can invade almost any dead woody tissue. It
is almost certain to invade twigs killed by fire blight. Early leaf infections often are seen in coneshaped
areas of the apple trees, with a dead twig or mummied fruit at its apex.
When the pycnidia become wet, spores ooze from them to become distributed by raindrops;
these may infect the leaves, the calyxes of blossoms or tiny fruit, and wounds in twigs and limbs.
Infections of fruit and wood may not become visible for several weeks. Throughout the growing
season, new infections occur during periods of wet weather. First fruit infections occur during
the bloom period, although these are not evident until mid-season or later. Throughout the
growing season, infections occur through wounds. Harvest injuries may become infected and the
fruit decay during or after storage. This is especially true with fruit harvested during a wet
period. Leaves are invaded through stomates ("breathing pores" of the leaf) and through injuries
such as caused by spray materials. Dead fruit spurs or twigs, especially those killed by fire
blight, are invaded. Entry points to limbs are through shoots killed by fire blight, pruning stubs,
and areas damaged by winter injury and sun scald.
While a distinct canker phase is not recognized on pear and quince, nor does black rot occur
on their leaves, dead twigs and fruit spurs are invaded. These, and mummied fruit hanging in the
tree, provide the source of spores for fruit rot. The origin and progression of fruit rot are similar
to that on an apple.
BLOSSOM END ROT
Hosts: Apple. The causal organism also causes a severe bunch rot of grape.
The disease, caused primarily by the fungus Botrytis cinerea, occurs on the blossom end of apple
fruit. The infection likely occurs during bloom, although it does not become visible until several
weeks later. The infected area is seen as a small, 1/4 to 1/2 inch diameter lesion next to or
including part of the calyx. The spot usually is brown in color, slightly sunken, and often has a
red border. A shallow, dry or corky rot develops in the flesh underneath the spot.
The disease appears to be more common in seasons of prolonged cool, wet weather during
and shortly after bloom. It has appeared most frequently on Delicious, Rome Beauty, and McIntosh.
Blossom end rot on fruit placed in storage often leads to moldy core. This is especially
true in Delicious.
Hosts: Apple and crabapple.
The disease is caused by a fungus, Phyllosticta solitaria, which can infect fruit, leaves and twigs.
Symptoms -- There are two types of leaf spots. The less frequent one occurs on the leaf between
the veins as a small, light-gray spot with a dark dot in its center. The more common leaf spots
occur on the veins, midribs, and petioles (leaf stems) as long, narrow, slightly sunken, lightcolored
lesions. These contain several dark dots, the fruiting structures (pycnidia) of the fungus.
When petiole infections are numerous leaf drop may result.
New shoot infections first appear similar to the petiole infections except they are longer and
are seen more easily. They occur at the junction of the petiole with the shoot (node) or between
the nodes. Once the lesion becomes established, it may continue to enlarge for 3 or 4 years,
becoming noticeably larger than the diameter of the normal limb. In this manner the organism
causing apple blotch becomes established in the tree.
Fruit infections vary in size from small, dark spots to large blotches that may cover much of
the fruit surface. Edges of the larger lesions are irregularly lobed with many radiating projections.
Large lesions often cause the fruit to crack.
Disease Cycle -- The causal fungus remains alive throughout the year in the twig and limb
cankers. The first infections in the spring occur about petal fall on leaves, young fruit, and new
shoot growth from spores oozing from the cankers. Secondary infections from spores produced
in the pycnidia can occur until late summer. Frequent rains and temperatures above 75o F. favor
Hosts: Apple, crabapple, pear, grape and chestnut.
This disease, caused by the fungus Botryosphaeria ribis, is most important on apple where it can
be seen as a distinct canker on twigs, limbs and trunks. Two types of fruit rot may occur, but
there are no leaf infections.
Symptoms -- New infections on twigs and limbs start to become evident by early summer. They
appear as small circular spots or blisters. When the blisters rupture, a watery substance spreads
over the bark around the area of infection. As the canker enlarges, it becomes slightly depressed.
The cankers stop enlarging in late fall. The following spring, some of those in the limbs will
continue to grow while others will not. Small, black pimples, the spore-containing structure of
the fungus, appear in rings on the smooth surface of the new cankers in late fall or early spring.
On older cankers, these may be present throughout the year. In addition to the wet spots of new
cankers, older ones can be distinguished by the scaling of the papery outer bark. Under the
surfaces of the cankers, the tissues are watery or slimy and browned. The cankers are not deep,
extending at most to the wood.
The fungus causes two fruit rots. One originates from external infections and the other appears
to start internally. The external rot first appears as the lenticels. The brown center becomes
slightly depressed. Meanwhile, under the spot, the flesh begins to decay in pockets that are cupor
egg-shaped. Once these decayed areas begin to enlarge rapidly, the lesion on the skin also
enlarges, often to one side of the original spot. The decayed areas run together and ultimately the
entire fruit is involved in a "soft rot." No fruiting bodies of the fungus can be seen on the fruit,
but a syrupy head of liquid may form on the skin. The skin of red apples may bleach during the
decay process and become a light brown color. Because of this characteristic the disease may be
referred to as white rot.
This external rot of fruit can be confused with black rot and bitter rot diseases. The decayed
apple flesh of black rot is firm and somewhat leathery, the surface of the spot is not sunken, and
pimple-like fruiting bodies eventually are present. Botryosphaeria rot is at first cup- or eggshaped
areas of decayed flesh. The rot is soft, the surface of the spot is slightly sunken and no
fruiting bodies form. Bitter rot causes cone-shaped areas of decay, the surface is sunken, and
concentric fungus of spores form on the surface.
Apples with no external sign of decay may break down rapidly after harvest or after removal
from cold storage. The rot is soft and droplets of a clear sticky liquid may form on the skin.
Fruiting bodies of the fungus can be seen under the skin if the apples are left at room temperature.
The origin of this internal decay is not known.
Disease Cycle -- The fungus overwinters in fruiting bodies on dead woody tissue. During spring
and summer rains, the spores ooze from these and are splashed to other parts of the tree. The
fungus invades dead wood, and fire-blighted twigs and branches especially. It also attacks living
twigs, branches, and trunks. Fruiting bodies are formed in all of these, providing a source of
spores for further infections. Fruit infections can occur at any time from the bloom period to
harvest. Infections in young apples usually are not evident until near maturity. The external rot
lesions are found most commonly on the sides of fruit exposed to high temperatures.
BROOKS FRUIT SPOT
Hosts: Apple and crabapple.
Caused by the fungus, Mycosphaerella pomi, Brooks fruit spot also is known by the name Phoma
fruit spot. The disease is found only rarely in well-sprayed orchards. When the cover sprays are
stopped too soon, or when trees are not well-pruned and sprayed, severe losses can occur. Certain
varieties, such as Rome Beauty, Stayman, Jonathan, and Grimes Golden are quite susceptible.
Symptoms and Disease Cycle -- The spots on the fruit are small, about 1/4 inch in diameter.
They are somewhat irregular in shape, slightly sunken, and often most numerous on the calyx
end of the fruit. On red fruit surfaces they are red to black in color; on green and yellow fruit
surfaces they are dark green. These spots may be quite inconspicuous at harvest. Unless the
infected fruit are placed in cold storage immediately after harvest, the spots increase in size,
become more sunken, and thus more visible.
The disease cycle is much like that of apple scab, except it begins later in the spring. About
the time of petal fall, ascospores are discharged from fallen leaves. Just how the fungus gets on
the leaves is not known as there is no evidence of the disease on leaves while they remain on the
trees. Rain and high humidity favor the spore discharge and infection of the fruit. Infections
continue until midsummer, although in decreasing numbers as the season progresses.
Hosts: Peach, nectarine, apricot, plum, cherry, apple and pear.
Successful stone fruit production depends on brown rot control. The fungus, Monilinia
fructicola, attacks the blossoms, twigs and fruit of peach, nectarine, apricot, plums, and cherries.
While blighted blossoms and twig cankers play an important role in the disease, the rot of ripening
fruit causes the greatest economic losses. The rot may occur while the fruit is on the tree, or
it may not become evident until after storage, in transit, or at retail outlets. Wet, humid weather
during the bloom period is ideal to initiate the disease in the spring. When this is followed by
wet, humid weather just before harvest, it almost insures a high incidence of fruit rot.
The fruit of apple and pear may become infected and decay. However, this disease is not as
common on these fruits as it is on stone fruits.
Symptoms -- Blossom blight - An unnatural wilting and browning of blossoms about the time the
petals fall are the first indications of blossom blight. The diseased blossoms appear much like
those that were not pollinated. However, when tipped with a finger, the diseased ones will seem
stiff and stuck to the pedicel (fruit stem), while those not pollinated are pliable and will fall
easily. In a few days the infected floral parts become covered with a powdery mass of gray
spores and the base of the flower becomes surrounded by a gummy ooze that sticks the flower to
Twig cankers - The first twig cankers of the season result from blighted blossoms. The
fungus grows from the base of the infected blossom to the pedicel and through it into the twig.
Here a somewhat elliptical canker is formed, surrounding the blighted blossom. It too becomes
gummy and gray tufts of fungus spores are scattered over its surface and around its edges during
wet periods. Some of these cankers may girdle the twig resulting in death of the portion beyond
the canker (twig blight). The leaves on blighted twigs appear bleached at first, then brown. They
often remain attached for several weeks.
Twig cankers also form when the fungus grows from the rotted fruit through the pedicels to
the twigs. These cankers have the same appearance as those resulting from blighted blossoms.
They are an important site for overwintering of the brown rot fungus.
Fruit rot - Brown rot on the fruit first appears as a small circular spot. Immature fruit resist
infection unless injured by hail or insect punctures, except when they touch a blighted blossom,
twig canker, or an infected fruit. Occasionally, the fungus will grow from an infected twig into
the fruit stem and then to the fruit. As the fruit nears maturity, the brown rot fungus easily can
penetrate the unbroken skin. The rot progresses rapidly at ordinary temperatures. In as few as 2
to 3 days the entire fruit may be rotted unless slowed by cold storage temperatures. The skin of
brown rot-infected fruit remains firm and unbroken. The rotted flesh is mushy-soft. The surface
may become covered with a gray powder of fungus spores. Some of the rotted fruit may remain
attached to the tree and become a hard mummy. During wet periods of early spring these may
produce spores. Other rotted fruit will fall to the ground and, unless they disintegrate completely
or become buried, the fungus can remain in them over winter and produce its sexual spores the
Disease Cycle -- Mummied fruit that dropped to the ground and became partially buried in trash
and soil develop small, vase-shaped, tan-colored fruiting bodies (apothecia) in the early spring.
Their broad tops are 1/8 to 1/2 inch in diameter and their stems are 1/2 to 1 1/2 inches long.
During the bloom period the sexual spores (ascospores) are discharged with force from the inner
surface of the vase to drift in air currents. Infection occurs in the blossoms during wet periods as
short as 3 hours when the temperature is as high as 70o F. When wet, the mummies hanging in
the trees and the surface of twig cankers, especially those resulting from fruit rot, may produce
tufts of gray spores (conidia) that also can infect the blossoms. In cherries and plums these are
the most common means of overwintering. The spores for the fruit rot phase of the brown rot
disease are produced on the blighted blossoms, new twig cankers, and newly infected fruit. Only
2 or 3 blighted blossoms in a tree, and their resultant twig cankers, can produce sufficient numbers
of spores for numerous infected fruit should there be wet weather just before or during
harvest. Often apparently sound fruit is harvested only to have it rot when it reaches retail outlets
due to infections at harvest or during pack-house operations.
Control -- All cultivars of stone fruits can be attacked by the brown rot fungus. Sweet cherry,
plum, and nectarine often suffer more severe losses than peach, apricot, and tart cherry. Most of
the peach cultivars developed in the last 20 years are less susceptible than many of the older
Most of the brown rot control efforts will need to be spent in sanitation practices and fungicides.
All fruit remaining on the trees after the last picking should be removed. This will reduce
the number of twig cankers in which the fungus may overwinter. Cutting out blighted twigs
when they are seen, but no later than normal leaf fall, also will reduce the overwintering sites.
During early spring pruning the trees should be thinned out to aid in rapid drying and thorough
spray coverage. Any remaining mummies should be knocked to the ground. A light cultivation
just as the first blossoms open will disturb the partially buried mummies and prevent them from
producing ascospores. The fungicide program must be designed to protect the opening blossoms
and the fruit, especially just before and during harvest.
Losses from brown rot in harvested fruit can be reduced by the rapid removal of field heat
from the fruit. This is done most efficiently by hydro-cooling where the volume of fruit warrants
the equipment cost. However, brown rot spores in dirty hydrocooling water can infect otherwise
sound fruit. Fungicides may be added to the water or to waxers in the pack-house line to disinfect
the surface of the fruit. Following removal of field heat from the fruit, they need to be kept
refrigerated until purchased for consumption or processed.
The fungus causing cherry leafspot, Coccomyces hiemalis, attacks the leaves, leaf stems, fruit,
and fruit stems of tart, sweet, and English Morello cherries. The disease is most severe on leaves
and may cause them to drop prematurely. When defoliation occurs before harvest, the fruit fail to
mature normally, being light-colored and low in soluble solids. The buds and wood become
susceptible to winter injury, which may show the next season as poor growth, dead spurs, and
Symptoms -- The disease first appears on the upper sides of leaves as tiny, red to purple, circular
spots. These enlarge to 1/8 to 1/4 inch in diameter, becoming red-brown to brown. By then, the
spots show brown on the undersides of the leaves and during wet periods tiny, whitish, felt-like
patches appear in their centers. These contain the spores (conidia) of the causal fungus. The
spots on sweet cherry leaves tend to be somewhat larger. Some of the spots may drop out,
leaving a shot holed appearance.
After the leaves become infected, they turn yellow and fall. This yellowing and dropping
often is confused with the cherry yellows virus disease, which may cause a heavy leaf drop in
June or early July. Leaves from yellows-infected trees are a mottled green and yellow with few,
if any of the spots characteristic of cherry leafspot.
Disease Cycle -- The fungus overwinters in diseased leaves on the ground. About bloom time, or
shortly after, the sexual spores (ascospores) mature and are discharged. They are blown to
young, expanded leaves where infection takes place through the stomates on the underside of the
leaf. These first infections often are so few in number they are overlooked. However, the conidia
from the felt-like centers of the spots on the undersides of leaves will be matured for 10 to 15
days after the first infections. They are spread by rains. Each succeeding wave of infection
becomes heavier and severe defoliation begins.
Control -- Rotary mowing the orchard after leaf drop in the fall and fall application of nitrogen, if
any is necessary, will hasten the decay of the leaves and reduce the numbers in which the fungus
can overwinter. A light discing around the trees just before bloom also decreases the ascospore
discharge of the cherry leafspot fungus, as well as the brown rot fungus. The main control of the
disease comes from the use of fungicides.
Hosts: Peach, nectarine, apricot, plum, cherry, apple, pear and quince.
This disease is caused by a bacterium, Agrobacterium tumifaciens. It can attack many different
plants as well as all fruit trees. Peach and Mazzard cherry rootstocks are especially susceptible.
The disease is common in tree fruit nurseries and eventually occurs on large numbers of trees in
Symptoms -- The disease is readily recognized by wart-like swellings or galls on the roots and
crown. Occasionally, the galls may be seen above ground on trunks or branches. Young galls are
light in color and cheesy in texture. They become dark and hard with age and may be 1/2 inch to
3 or 4 inches in diameter. When the galls are numerous or when they are located on major roots
or the crown, they may disrupt the flow of water and nutrients. The trees then will show reduced
growth, an unhealthy appearance, and possibly nutritional deficiency symptoms.
Disease Cycle -- The bacteria causing crown gall are distributed widely in many soils. They can
attack a very large number of different kinds of plants. In addition, soils may become contaminated
by planting nursery stock dug from contaminated fields. Once in the soil, a few of the
bacteria survive for several years in the absence of host plants.
The bacteria must have a wound in order to enter the plant. The more common wounds
serving as entry sites are those made during the nursery digging and tree planting operations,
tillage equipment, and the injuries made by root-feeding insects and nematodes. Once a gall is
initiated, the bacteria play no further part in its growth and development. In this respect, crown
gall has been called a plant cancer. Secondary galls may develop a considerable distance from
the initial infection. These too are formed in the absence of the crown gall bacteria, apparently
due to a tumor-inducing substance, produced at the site of the original infection.
Soil fumigation can play an important role in control of this disease, especially in nurseries.
Hosts: Most common on apple and pear.
The fungus-inducing crown rot is caused by Phytophthora cactorum. Several other disorders
have similar symptoms and often are termed collar rot or crown rot in error. The more common
of these are fire blight when it enters rootstock suckers, a drought-induced winter injury, a south
or southwest side winter injury brought about by fluctuating winter temperatures, stem grooving
virus, brown-like decline virus, and rootstock-scion incompatibility.
Symptoms -- Initially cankers are produced underground in the area of root attachment to the
crown, the below-ground portion of the trunk. From this point the cankers may spread out in the
roots and up the trunk to or above the soil line. If the scion cultivar is susceptible, such as
Grimes Golden, the cankers may progress past the bud union. Otherwise, they do not and often
they cease their upward growth at the soil line or, at the most, 4 to 6 inches above it. Tissues
under the bark of young cankers turn brown and become gummy. Above the soil line the outside
of the bark may appear wet. As the canker ages, the bark often cracks at the margin and becomes
slightly sunken, especially when callus tissue is laid down around the lesion. The margins of the
canker are usually lobed. When the bark just outside the lobes is cut away it is not unusual to
find pockets of brown tissue that are not evident externally.
The cankers girdle the roots and the trunk. Tree top symptoms are reduced growth and yellow
to reddish-purple leaves. The trees may decline for a period of 2 to 3 years or they may die
during the season aboveground symptoms appear. The disease is most active on trees 3 to 10
years of age.
Disease Cycle -- The fungus causing crown rot lives in the soil. It can survive long periods of
unfavorable conditions in thick-walled spores. It has a wide range of hosts and presumably may
be present in most soils suitable for fruit trees. Once trees are infected, it can survive in the
margins of the cankers as well. In the fall and spring, during periods when soils are saturated
with water, swimming spores are formed. These move in the water film to new potential points
Control -- Careful selection of orchard sites with respect to internal drainage oftentimes prevents
damage by collar rot. Draining wet areas of otherwise good orchard sites should be considered.
There are susceptibility differences in rootstocks. Apple rootstocks of Melba, McIntosh, and
Wealthy seedlings appear resistant. Delicious seedlings generally are moderately resistant. Of
the colonial rootstocks, EM IV, EM IX, and EM 26 seem resistant; EM II, EM VII, and MM III
are susceptible, and MM 104, MM 106, and EM XI are very susceptible.
Most pear rootstocks are susceptible to collar rot. These often include seedlings of domestic
and imported French and Old Home pears (Pyrus communis), and Japanese pear (P. serotina (P.
pyrifolia)). Quince and P. calleryana root-stocks are resistant.
By the time aboveground symptoms show, the disease is often too well established to eradicate
it and save the tree. However, if more than half the circumference of the tree is alive,
inarching one-year-old resistant seedlings may be successful. This procedure is the same as that
applied to trunks injured from other causes. Chemical control has been but partially effective
when applied after infections occur. It likely would be more effective if applied as a preventative
Hosts: Apple, crabapple, pear and quince.
Fire blight is a destructive disease of apple and quince, and is the most serious pear disease in the
eastern United States. The causal bacterium, Erwinia amylovora, can attack some 75 species of
plants of the rose family. In addition to the above pome fruits, fire blight occurs frequently on
pyracantha, spirea, hawthorne, and mountain ash. On fruit trees, the disease can result in the
death of blossoms, fruit, shoots, limbs, and tree trunks.
Symptoms -- The disease often appears first in the spring as blossom, fruit spur, and new shoot
blight. Infected blossoms wilt rapidly and turn light to dark brown. The bacteria may move
through the pedicel to the fruit spur and out into the leaves. Here they follow the midrib and
main veins, which soon darken. The leaves wilt, turning brown on apple and quince and dark
brown to black on pear. The blighted leaves remain attached for much, if not all, the growing
season. Some remain even after normal leaf fall.
Shoot blight begins with the infection of the young, succulent growing tip. It may occur any
time during the season while the shoots are still growing when environmental conditions are
optimum for the disease. The leaves wilt rapidly, turn dark, and remain attached as in the case of
spur blight. A characteristic symptom of shoot blight is the bending of the terminal growth into
the shape of a shepherdís crook. Pearly or amber-colored droplets of bacterial ooze often are
present on diseased blossoms, fruit, and leaf stems, on succulent shoot stems, and on the exterior
of infected fruit. These droplets contain millions of bacteria, which cause new infections.
The fire blight bacteria can move from blighted spurs and shoots into the larger limbs and tree
trunks. Infected branches may be girdled resulting in the loss of the entire branch. Suckers at the
base of trees often are invaded. These may blight back to the trunk or rootstock resulting in the
loss of the entire tree in one season. This is especially true on susceptible pears and certain
clonal apple rootstocks.
Fire blight cankers occur on small to large limbs, trunks, and even roots. They often begin at
the bases of blighted spurs, shoots, and suckers. The cankers are slightly sunken areas of various
sizes, surrounded by irregular cracks. Active blight cankers are characterized by an amber or
brown exudate on their surfaces or on the bark below them.
The fire blight bacteria may invade the fruit, which becomes water-soaked. Droplets of
bacterial ooze will appear on the surface. The fruit later becomes leathery, turns brown on apple
and black on pear and quince, shrivels, and usually remains attached to the fruit spur.
Disease Cycle -- The bacteria overwinter in the margins of cankers on branches and trunks. Once
the temperature reaches about 65o F, they begin to multiply and appear on the outside of the
cankers in drops of clear to amber-colored ooze. The bacteria are spread to blossoms or succulent
new growth by insects or rain. If the temperature is 65o F or above, and the relative humidity
is 60% or more, or there is rain, new infections can occur. At 75o F, blossom blight and shoot
blight will be evident in 4 to 5 days. Bacterial ooze appears on the new infections soon after the
symptoms. This provides additional sources of bacteria for new infections. In early to midsummer,
during prolonged periods of muggy weather, blighted shoots and spurs, infected fruit, and
new branch cankers all may have droplets of ooze on them.
The bacteria enter the flowers primarily through natural openings. Wounds and natural
openings are important entry points to leaves, shoots, and fruit. Insects with piercing mouthparts,
such as aphids and leafhoppers, may transfer the fire blight bacteria directly into susceptible
tissues. The feeding wounds of other insects can serve as entry points. Wounds from hail often
lead to a severe outbreak of fire blight. Any fresh wound can serve as an entry point.
Hosts: Peach and nectarine, primarily.
Leaf curl on peaches and nectarines is caused by the fungus Taphrina deformans. The fungus
mainly attacks the leaves, but on occasion new shoot growth and fruit may become infected.
Flowers may be invaded, but they drop soon after opening showing no distinctive symptoms.
Symptoms -- The infected leaves show symptoms soon after they unfold. They become crinkled
or puckered, thicker than normal with a leathery feeling, and light-green, yellow, red, or purple in
color. The entire leaf or any part of it may be infected. In severe outbreaks of leaf curl, nearly all
the leaves on a tree are infected. More often, only the leaves of occasional buds scattered
throughout the tree will be diseased. By late spring or early summer, their upper surfaces turn
grayish and take on a powdery appearance due to the production of fungus spores. Dry, warm
weather soon causes the leaves to turn brown to grayish-black and fall from the trees.
Infected new shoots grow slower than normal, are somewhat swollen, and are pale yellow in
color. Severely infected young fruit become misshapened and seldom remain on the tree.
Lightly infected fruit mature normally, although they will have one or more wart-like, irregular
growths on them. These have no fuzz and are usually reddish in color. They are similar to the
irregularities that occur along the suture of some cultivars, for example the Rio Oso Gem and
Disease Cycle -- The spores produced on the infected leaves are blown or splashed to the newly
forming buds, the cracks and crevices of the twigs and bark, and the waxy coating of the new
shoots. Here they live harmlessly, occasionally producing new spores when weather conditions
are suitable. These too are spread throughout the tree by wind or rains. In the early spring, just
as the buds begin to swell, should a spore become lodged in the bud, it may germinate. Thus, all
the tiny young leaves may be infected before they unfold, as may the flower or fruit. New shoot
infection results from diseased terminal or lateral buds.
Only young, tender tissue can be infected so there is little secondary spread of the disease.
Cool, slow growing conditions that prevent rapid bud swell and leaf growth plus wet periods are
necessary for several outbreaks. These can occur even though the disease has not been noticed
for several seasons, as the fungus is nearly always present on the waxy coating of the twigs.
NECTRIA TWIG BLIGHT
Hosts: Apple twigs.
Nectria twig blight is a minor disease that breaks out occasionally. It needs to be recognized
since it appears much like fire blight, a very serious disease of apples and pears. The chemical
controls for fire blight would be wasted on nectria twig blight, caused by the fungus Nectria
Symptoms -- In early summer the leaves and shoots on infected twigs wilt and turn brown. A
close examination will show a canker has girdled the twig at the point of shoot growth initiation.
Most often this will be located at the base of the previous seasonís cluster-bud. Rome Beauty,
which is characterized by enlarged cluster-bud bases, is very susceptible. In midsummer, a few
pink or coral fruiting structures of the fungus may appear in the cankered area.
Hosts: Peach, nectarine, apricot, cherry, plum, apple, pear and quince.
Most of the many species of nematodes living in the soil do not attack plant roots. A few can
injure fruit trees seriously when their populations become sufficiently high. The only symptom
of attack likely will be reduced growth and vigor of the trees. Nematodes may be important as
vectors of disease, such as is the case in transmitting the Prunus stem pitting virus. Injuries made
by nematode feeding can provide the entry sites for other disease organisms, such as the bacteria
that cause crown gall and the fungi that cause Verticillium wilt.
Control -- Preplant and postplant soil fumigants may be used to control nematodes. Growers
should contact their state extension service about a soil analysis to determine the kinds of populations
of nematodes in orchard soils. Various suggested treatments then can be made, depending
on the individual situation.
Hosts: Peach, nectarine and apricot.
Caused by the fungus Cladosporium carpophilum, the disease known as peach scab occurs on
peach, nectarine, and apricot. It often is severe on the fruit.
Symptoms -- Scab occurs on fruit, leaves and twigs. It is of primary concern on the fruit. Here
the disease is seen first as tiny, poorly defined, olive-black spots on the skin. The disease normally
becomes evident about 10 to 12 weeks after shuck fall and new infections may continue to
appear until harvest. The spots often are most numerous on the stem end of the fruit. They
enlarge very slowly, become distinct, circular, and black. They eventually reach 1/8 to 1/4 inch
in diameter. The lesions on apricot fruit are more brown than black. Where the infections are so
numerous as to run together, the fruit may crack during its final swell before harvest. The spots
themselves are superficial, never penetrating below the epidermal layer of cells. They can be
distinguished from bacterial spot lesions as these do penetrate slightly below the epidermis.
In the orchard, twig infections are important only as overwintering sites for the fungus. These
show on the current seasonís growth as circular to oval, tan to purplish-red spots. The diseased
area is raised slightly. When they are numerous, the bark may crack and the twig becomes
dehydrated and killed. This is most likely to happen on trees just dug from nursery fields.
Leaves may be attacked, but these are seldom noticed. The infected areas become olive-green,
diffused, small blotches located between the main veins. Long, narrow, brown lesions may
develop on the midrib of the leaf. When these are numerous some defoliation will occur.
Disease Cycle -- The fungus overwinters in the twig lesions. About the time of the shuck fall,
and up to 6 weeks after, spores are produced in the centers of these lesions. They are washed or
splashed to the fruit, leaves, and new shoot growth. Some 6 weeks or more after infection takes
place, the lesions become evident. This long delay between the time of infection and appearance
of symptoms often puzzles growers when they attempt to evaluate the reasons for an outbreak of
PEAR LEAF BLIGHT AND FRUIT SPOT
Hosts: Pear, quince and apple.
The name of this disease should not be confused with the fire blight or leafspot diseases of pears.
Leaf blight and fruit spot are caused by the fungus Fabraea maculata. The fungus infects leaves,
fruit, and shoots of pear and quince and the leaves of apple. The disease can build up rapidly,
even in orchards where it has not been a problem. When conditions favor the disease, and it is
not controlled, pear trees may become defoliated in a few weeks.
Symptoms -- Leafspots first appear as small purple dots on the leaves nearest the ground. They
increase in size to about 1/4 inch diameter circular spots, becoming purplish-black or brown in
color. A small black pimple appears in the center of the spot. When the leaf is wet, a gelatinous
spore mass oozes from the pimple giving the spot a creamy, glistening appearance. Each leaf
may have dozens of spots resulting in extensive defoliation. Fruit lesions are much like those on
the leaves, but they are black and slightly sunken. They may be so numerous as to run together
and cause the fruit to crack.
The lesions on the twigs occur on the current seasonís growth. They are purple to black in
color with rather indefinite margins. The lesions may run together forming a superficial canker.
Early defoliation results in small fruit, weak bud formation, and fall blossoming. Infected
fruit have no sale value and often are cracked and misshapened.
Disease Cycle -- The sexual spore stage develops on fallen, overwintered leaves. Conidia,
asexual spores, also may develop in the spots on overwintered leaves, or they may be produced in
the previous seasonís shoot infections. Often, the first infections do not occur until mid-June to
the first of July. Secondary infections begin about one month later and reoccur throughout the
season during periods of rains.
Hosts: Pear, quince, apple.
The pear leafspot fungus, Mycosphaerella sentina, infects the leaves of pear, quince, and occasionally
apple. Numerous leafspots can result in defoliation. Fortunately, this does not occur
often before fall, except in nurseries.
Symptoms -- Mature leafspots are recognized easily by their grayish-white center with sharply
defined margins. They appear first on the upper leaf surface as small, brown lesions and enlarge
to 1/8 to 1/4 inch in diameter. The borders become dark brown and small, black pimples appear
in the center of the spots.
Disease Cycle -- Sexual spores are produced on the overwintered, fallen leaves. They are carried
by air currents to the newly-formed leaves. About a month after infection, new spores are produced
in the centers of the grayish-white leafspots, from which they are washed by rains to other
leaves. The peak of these secondary infections usually occurs in late summer or early fall.
This disease resembles apple scab in nearly all respects, and is caused by the closely related
fungus Ventruia pirina. Pear scab is not particularly common, although it is very destructive
when it occurs. The symptoms and disease cycle are so similar to apple scab, they need not be
repeated here. A major difference is the frequent appearance of pear scab on twigs, where it can
overwinter and initiate new infections in the spring. Leaf infection of pear is not as common as
apple scab on apple leaves.
This disease of plums and prune-type plums is caused by the fungus coccomyces prunophorae.
The fungus, its life cycle, and the disease are very similar to the cherry leafspot disease. The
spots on plum leaves tend to be smaller and severely infected leaves often have a tattered appearance.
Unlike cherries, heavy plum leaf infection often is followed by a heavy fruit drop.
Most cultivars are susceptible to the disease, so fungicide sprays and the sanitation suggestions
presented in the cherry leafspot discussion are needed for control. As the overwintered
spores on the leaves are ready for discharge about the time of shuck fall, light discing should be
done just before then.
There are a number of diseases of stone fruit caused by fungi similar to the leaf curl fungus.
Commercially, the most important one in the Northeastern U.S. is a disease of American-type
plums known as plum pockets or bladder plum. It is caused by Taphrina communis.
Symptoms -- First evidence of the disease on the fruit are small, white blisters. These enlarge
rapidly, soon involving the entire fruit. The fruit becomes spongy and the tissues of the seed
cavity wither and die. The fruit enlarges rapidly, becoming up to 10 times normal size, reddish in
color, and so misshapen that they hardly are recognized as plums. As the spongy interior dries
up, the color of the plums becomes velvety-gray due to the production of spores on their surface.
New shoots and leaves usually are infected as well as the fruit. The shoots become much
thickened and often curled or twisted. Diseased leaves are thickened and curled as described for
Disease Cycle -- Spores of the fungus overwinter on the twigs. During cool, wet periods or early
bloom, they may be splashed to the opening buds where infection takes place. With the development
of the ascospores giving the infected fruit a velvety-gray appearance, the disease cycle is
POWDERY MILDEW OF APPLE
Host: Wild and cultivated apple and crabapple
The causal fungus (Podosphaera leucotricha) attacks buds, blossoms, leaves, new shoots, and
fruit. Powdery mildew interferes with the proper functioning of the leaves, reduces shoot growth,
reduces fruit set, and produces a net-like russet on the fruit of some cultivars. It is often a serious
problem in apple nurseries.
Symptoms -- The first sign of the disease in the spring is a 3 to 4 day delay in the opening of
infected buds. The leaves and blossoms of these buds soon become covered with a white to light
gray powder, the spores of the powdery mildew fungus. The flowers do not develop normally,
are likely to be greenish-white in color, and will set no fruit. The first symptoms on leaves of
new shoot growth are felt-like, white patches of the fungus on the margins and lower surfaces.
Infected leaves curl upward and soon become covered with a powdery coating of fungus spores.
New infections of succulent leaves and growing shoots cause the entire shoot to be reduced in
size. By midsummer, these leaves and shoots may turn brown. The fruit of some cultivars may
show a net-like russet as a result of infection that occurred during the blossoming stage.
Disease Cycle -- The fungus overwinters as mycelium (fungus threads) inside infected buds. As
these buds open in the spring, all of their parts become covered with a powdery coating of spores.
The spores are easily windblown and they serve to infect new leaves, fruit, and shoots. Fruit
infections occur during and shortly after the blossom period. Leaf and shoot infection may
continue as long as shoot growth continue. Bud infection occurs from the time the buds begin to
form until they are matured for overwintering. Disease development is favored by the absence of
rain, but by high humidities and temperatures of 65o to 80o F.
Chemical Control -- Protective mildew sprays must begin with the delayed opening of the infected
buds. Where the disease is severe, mildew fungicides need to be continued until new
shoot growth stops or about the fourth cover spray.
POWDERY MILDEW OF CHERRY AND PLUM
Hosts: Cherry and plum.
The disease is caused by Podosphaeria oxyacanthae, one of the common species of the powdery
mildew group of fungi. This same fungus has been reported to cause powdery mildew on peach,
apricot, apple, pear, quince, persimmon, and a few ornamental plants. This discussion is limited
to the disease on plums and tart and sweet cherries.
Symptoms -- The fungus attacks leaves and twigs producing symptoms much like powdery
mildew on apple. Infected leaves curl upward. Newly developed leaves on new shoot growth
become progressively smaller, are generally pale in color, and somewhat distorted. New shoot
growth is shorter than normal. By mid-season the whitish fungus growth can be seen growing
over the leaves and shoots, sometimes in patches and other times covering most of the new
growth. These symptoms are especially common in nurseries.
Disease Cycle -- The fungus may overwinter on diseased, fallen leaves, but it does so more
commonly in infected buds as in the case of apple powdery mildew. When infected buds expand
in the spring the new growth becomes overrun by the fungus. Much of the visible white growth
consists of conidia, which are spread by wind to other new leaf and shoot growth. Warm temperatures
without rain, but with sufficiently high humidities for morning fog or dews are ideal for
rapid increase of the disease.
POWDERY MILDEW OF PEACH, NECTARINE AND APRICOT
Hosts: Peach, nectarine, apricot and some woody ornamentals, especially roses.
This disease, sometimes called rose mildew, is not often serious. The causal fungus,
Sphaerotheca pannosa, attacks the leaves, twigs, and fruit.
Symptoms -- On fruit, the disease first appears as round, whitish spots two to four weeks after
shuck fall. The spots increase in size until much of the fruit is involved. The white spots are due
to the fungus mycelium and its spores. About the time of pit-hardening, the skin of the fruit
under the spot turns pinkish and the fungus and its spores disappear. Eventually the skin becomes
leathery or hard, turns brown, and may crack.
Diseased leaves often fail to unfold normally, while the ones on new shoot growth become
narrow, strap-like, and distorted. New shoots are shorter than normal and distorted. The white
mycelium and spores of the fungus may cover the infected leaves and shoots or they may appear
in whitish patches.
Disease Cycle -- The fungus overwinters on shoots infected the previous season. Quite likely it
hibernates behind the leaf bud scales. Flower buds on infected shoots often do not survive the
winter. As the leaf buds expand in the spring the young leaves become infected and the spores
produced on these leaves serve to infect the young fruit, new shoot growth, and newly expanding
PRUNUS STEM PITTING VIRUS
Hosts: Peach, nectarine, apricot, cherry and plum.
Prunus stem pitting can attack all stone fruit trees, resulting in a girdling of the trunk and eventual
death of the tree. Diseased trees do not recover.
Symptoms -- Leaves of infected trees show the first external evidence of the disease, usually in
late summer. On a single or several shoots, one or more branches, or rarely much of the tree, the
leaves become pale green, cup upward along the midrib, turn various shades of yellow through
red to purple and then begin to drop from the tip back towards the main branches. The following
season the tree will appear weak, growth will be stunted, many of the flower buds will have been
killed during the dormant season. The few fruit that set will be misshapen; they will ripen
prematurely and have an insipid taste. Eventually, the infected trees die or they may break off
near the groundline during heavy winds.
The above symptoms are, at least in part, similar to those of trees girdled from other causes.
To further identify Prunus stem pitting it is necessary to remove a strip of bark from a few inches
above the ground to 6 to 8 inches below the groundline on the trunks of suspect trees. On infected
trees, the bark and sapwood will be much thicker than normal. The wood will have elongated
indentations, pits or swellings. The degree of pitting depends on the kind of stone fruit
tree, its scion and rootstock, and the stage of disease development.
A poorly developed root system results from severe virus infection of trees. Often many of
the roots will be on one side of the crown. When the trees are pulled, socket-like depressions
remain in the crown where the roots break away. Trees broken over reveal a serious disorganization
of the woody tissues. This disorganization results not only in a structurally weak tree, but
also produces the girdling effect resulting in the above ground symptoms.
Cause -- Prunus stem pitting is caused by a virus of the tomato ringspot group. The virus is soilborne
and is transmitted to healthy trees by a species of dagger nematode. Soil fumigation plays
an important role in the control of this virus disease, both in nurseries and orchards.
Hosts: Peach, nectarine, sweet cherry and plum.
This disease, caused by the "bread mold" fungus Rhizopus nigricans, can be very destructive on
harvested fruit. While it can develop in hail-injured or cracked fruit on the tree, it is most common
during storage, transit, and at the marketplace. Peach, nectarine, sweet cherry, and plum,
are most susceptible.
Symptoms -- The beginning of the rot is much like that of brown rot - a small, brown, circular
spot. There is a detectable difference. The skin of Rhizopus rot-infected fruit slips readily from
the underlying flesh, while the skin of brown rotted areas is tough and leathery. At normal
temperatures, the small spots of Rhizopus rot enlarge rapidly and can involve the entire fruit in
24 to 48 hours. A white, whiskery, mold appears on the surface of the infected fruit, spreading to
nearby fruit and the walls of the container. By this time the fruit often begins to leak and to have
a vinegar-like odor. Finally tiny, black, spherical structures are produced on stalks above the
white mold growth. Each of these contain thousands of spores (sporangiospores) that are released
to float in the air. At this stage the mold appears mostly black.
Disease Cycle -- The Rhizopus rot fungus occurs on all decaying vegetation, including ripe fruits
and vegetables. When environmental conditions are not favorable, it produces thick-walled
zygospores that can withstand long periods of cold and drying. These are present on dead vegetation,
in used fruit containers, and in pack-houses and storages. Thus, some type of spore of
the Rhizopus rot fungus is ever present where fruit are handled.
An injury through the skin of the fruit must be present for the first infections to occur. Injuries
as tiny as the prick of a pin are sufficient. In packed fruit, or clustered ripe fruit on the trees, the
fungus can spread over the uninjured skin from an infected neighbor and eventually causes a rot.
High temperatures and humidities favor the rapid growth of the fungus and decay of fruit.
Control -- Prevention of skin cuts and punctures during harvest and packing is the prime consideration
in control of Rhizopus rot. Clean containers and good housekeeping in the pack shed and
storage will aid greatly in reducing the spore population. Quick removal of the fruit field heat
drastically slows the advance of decay as does maintaining the fruit under refrigeration until it is
sold to the consumer or processed. A fungicide may be applied to the fruit prior to harvest if they
are not to go through a water system in the packhouse. Or, it can be added to the water pump,
hydrocooler, or water. To be effective, the water must be changed frequently.
There are three rust diseases. The most common is cedar-apple rust. All three must spend part of
their life cycle on red cedar. Only the cedar-apple rust is described, as the others are somewhat
Hosts: Cedar-apple rust - leaves and fruit of apple and crabapple.
Hawthorne rust - leaves of pear, hawthorne, apple and crabapple.
Quince rust - leaves and fruit of quince; fruit of pear, apple and crabapple.
The diseases can cause economic loss in several ways. Severe leaf infection and defoliation may
result in trees becoming susceptible to winter injury. Severe defoliation will reduce the size and
quality of the fruit. Infected fruit will be deformed, sometimes very severely.
Symptoms -- On leaves, cedar-apple rust, caused by the fungus Gymnosporangium juniperivirginianae,
first appears as small, pale-yellow spots on the upper surface. The spots enlarge to
about 1/8 inch in diameter. Eventually, tiny black, fruiting bodies called pycnia, become visible
in these spots. Often a number of orange-yellow protuberances, called aecia, are produced in
each spot on the underside of the leaf. Infected leaves may remain on the tree, or they may
become yellowed and drop.
Fruit lesions appear on the blossom (calyx) end. They are somewhat like the leaf lesions, but
much larger. They often cause the fruit to be disfigured or to develop unevenly. Aecia, like those
on the undersides of leaves, may develop in the fruit lesions.
Light-brown to reddish-brown galls form on the branches of red cedar. When they are dry and
hard they may be 1/2 to 2 inches in diameter and are known as "cedar apples." The surface of the
galls are covered with depressions much like those on a golf ball. In the spring, when the "cedar
apples" become wet, a yellow, gelatinous horn (telial horn), up to 2 inches long, protrudes from
Disease Cycle -- Spores from the gelatinous telial horns of the galls on red cedar are easily windborne.
These infect the apple and fruit. Spore discharge begins about the pink stage of apple
bloom and is usually completed in a few weeks. Following a few wet periods, the cedar galls
die. Spots on the apple leaf can be seen about 10 days after infection. Visible fruit infections
require a somewhat longer time.
The aecia on the underside of the apple leaf or on the fruit lesions produce spores that are
again windblown. These may be carried back to the red cedar. After lodging in the leaf axils or
in crevices on the cedar twigs, they germinate, infect the twig, and produce tiny galls the following
spring. One year later, these galls are capable of producing gelatinous horns bearing spores
that can infect the apple.
The cause of rusty spot is uncertain, although many plant pathologists believe it to be the apple
powdery mildew fungus.
Symptoms -- Rusty spot is recognized only on the fruit. The earliest symptoms are small,
orange-tan areas. This symptom is due to a change in the color of the fuzz or hairs on the fruit.
These first spots may become noticeable three to four weeks after shuck fall. The discolored area
enlarges slowly, and the older discolored hairs begin disappearing, leaving a fuzzless, smooth,
center spot surrounded by a nonuniform band of orange to tan hairs. Finally, the spots become
quite spread out leaving brownish or reddish centers of hard, smooth skin that appear somewhat
like a bruise from a limb rub. The highly red cultivars, or on the very red cheek areas of others,
may show little evidence of the disorder once the peaches are washed and brushed after harvest.
Less highly colored fruit often remain quite unattractive.
Disease Cycle -- Since the cause of rusty spot is not clear, no disease cycle can be given. New
spots on the fruit may continue to appear up to the time of harvest. There does not seem to be
any relationship between weather conditions and the number of diseased fruit.
Host: Apple and pear.
This common rot of stored apples and pears is caused by the fungus Penicillium expansum.
Other names of the disease are blue mold, bin rot, and Penicillium rot. In addition to losses from
the rot, sound fruit in the same container may absorb a moldy odor and flavor from the decaying
Symptoms -- Soft rot first appears as soft, watery spots on the outer surface of the fruit. These
spots begin around injuries or lenticels. The decayed flesh is light brown, soft, and watery.
Rotted fruit have a characteristic moldy odor and flavor. When the relative humidity is high,
grayish-blue, cushion-like masses of spores may appear on the fruit surface - thus the name blue
Disease Cycle -- The spores of the soft rot fungus are present most everywhere. They can survive
long periods of unfavorable conditions. Bulk bins, field crates, packhouse lines, and storage
rooms usually are contaminated. Injuries to the fruit, especially during picking and handling
operations, are the primary points of entry. At ordinary temperatures, infected fruit can rot in two
weeks or less.
Control -- Preventing injuries to the fruit during picking and handling and the rapid movement of
harvested fruit into cold storage are essential to control of soft rot. Picking bins and boxes can be
disinfected with fungicides or steam. Packing line equipment and storage rooms should be
cleaned. Clean water should be maintained in water dumps and anti-scald solutions. Disinfectants
or fungicides can be used in fruit dips and combined with wax applications.
SOOTY BLOTCH AND FLYSPECK
Hosts: Apple, crabapple and pear.
These are separate diseases but normally both are present on the same fruit. They cause only
surface blemishes that detract from the appearance of the fruit. Sooty blotch will shorten the
storage life of fruit due to increased water loss. Sooty blotch is caused by the fungus Gloeodes
pomigena and flyspeck by Microthyriella rubi.
Symptoms and Disease Cycle -- Sooty blotch appears as sooty areas on the fruit surface. Their
outline is not definite and the smudge can be removed by rubbing vigorously. The sooty areas
are olive-green to black in color. Flyspeck looks like true flyspecks in groups of 10 to 50 or
Both fungi overwinter on the twigs of numerous wild hosts as well as apple and pear. The
spores are windblown throughout the orchard. Cool temperatures and rains or high humidity are
ideal for infection. These diseases usually appear on fruit late in the season.
Hosts: Most common on pears, but can occur on all tree fruits and tree nuts.
Sooty mold fungi of the genus Capnodium cause an unsightly blackening over the surface of fruit
and leaves. It occurs on many plants and most frequently on pear of the various tree fruits.
These fungi live on honeydew excreted by some insects such as aphids, psylla, and white flies.
When only a few insects are present on the host plants, and thus a small amount of honeydew,
sooty mold appears in spots. When insect secretions are abundant, the surface of leaves and fruit
may have nearly a continuous coating of the black, tissue-paper thin layer of sooty mold.
Control of sooty mold is directed against the insects producing honeydew.