Click here for pesticide overview of all apple diseases.
Click on any disease below to go to that disease on this page.
Diseases (Berkett, 2000)
Apple Scab: Apple scab,
caused by the fungus Venturia inaequalis, is the most important disease of apples
in Vermont. This is because our humid, temperate climate and frequent rains, particularly
in spring, provide favorable conditions for infection by the fungal pathogen and our major
cultivars are highly susceptible to scab. The disease can be devastating, causing
reduction in fruit quality and yield. Severe foliar infection can lead to premature
defoliation. The key to managing scab is to prevent primary infections by ascospores in
the spring. By successfully preventing or limiting the development of primary lesions, the
threat of continued infection by conidia is reduced. There are six scab management
strategies: (1) Prevention, which includes planting scab resistant cultivars and cultural
practices to reduce inoculum and enhance the efficacy of fungicides; (2) Delayed
First-Spray Strategy, which involves predicting the orchard’s level of
"scab-risk" by a sequential sampling procedure and using an action threshold to
determine when to begin a fungicide program; (3) Protection Strategy, which is designed to
maintain a protective fungicide residue barrier on or in leaves and fruit; (4)
Post-Infection Strategy, which is designed to apply fungicide when there has been an
infection period; (5) Four-Spray-SI Schedule, which involves four applications of a
sterol-inhibiting fungicide combined with a protectant beginning approximately 1-3 weeks
after green tip; and (6) Conidia Suppression, which involves using fungicides to suppress
conidia production and further secondary infections. An array of fungicides are essential
to manage apple scab effectively and address resistance concerns.
Bitter Rot: This disease
is a summer disease of apple fruit caused by the fungus Glomerella cingulata. It is
considered a more serious disease problem in southern apple growing regions of the U.S.
but extensive damage can develop rapidly in New England during a period of prolonged warm,
wet weather if inoculum sources are present. At the optimum temperature for infection (79
F), infection can occur with a wet period as short as 5 hours. Fruit lesions appear as
brown, slightly sunken spots. Bitter rot lesions expand most rapidly at a temperature of
86 F. Severely infected fruit become shriveled and persist on the tree as mummified fruit.
Control of bitter rot is based on sanitation and protectant fungicide sprays. Removal of
mummified fruit and dead wood on the tree or on the ground is important. Shoots killed by
fire-blight can be colonized by the bitter rot fungus and serve as an inoculum source
during that same growing season. The EBDC fungicides are the most effective materials for
preventing bitter rot, but their label makes them unavailable for use. Of the other apple
fungicides, only captan has a "good" rating for bitter rot prevention.
Black Rot: Black rot is
caused by the fungus Botryosphaeria obtusa. The economic impact of the disease
varies from orchard to orchard. The name black rot refers to fruit rot or canker symptoms,
while leaf-spot symptoms are called frogeye. In addition, the fungus can cause a core rot
around the seed cavity in developing fruit. Frogeye symptoms first appear about 1 to 3
weeks after petal fall. Leaf infections first look like small purple flecks which rapidly
enlarge into circular lesions about 1/8" – 1 /4" in diameter. The lesions
resemble "frog eyes" in that they retain a purple margin and have a light
brown/tan center. Fruit (i.e., sepal )infections can occur as soon as bud scales begin to
loosen. These early infections can result in blossom-end rot later in the season.
Infections of young fruit after petal fall begin as reddish flecks but then develop into
purplish pimples which do not enlarge into dark brown necrotic areas until the fruit begin
to mature. Additional infection of mature fruit result in black, irregularly shaped lesion
surrounded by a red halo. As these lesions expand, they are characterized by alternating
brown and black concentric bands. The flesh of the decayed area remains firm and leathery.
The core rot caused by the black rot fungus is typically "moldy" around the seed
cavity, and may cause fruit to redden and drop 3 to 6 weeks early.. Infected areas of
branches and limbs are reddish brown and slightly sunken. Cankers can expand to several
feet in length. Fire-blight cankers and cold-damaged tissue are rapidly colonized by the
black rot fungus. Fruiting bodies (pycnidia) are abundantly produced on dead bark, dead
twigs, and mummified fruit. Ascospores and conidia are released when it rains throughout
the growing season. Infected leaves and fruit are often found below mummies and old fire
blight cankers. Removing cankered wood, mummified fruit, and chopping or removing pruned
wood are important steps in the management of this disease. If a disease problem persists
after implementing these sanitation tactics, then multiple applications of captan or a
benomyl or thiophanate-methyl in combination with captan may be needed from after petal
fall through mid-to late- August to prevent fruit infections.
Calyx End Rot and Dry-Eye Rot: Sometimes lumped together
and called blossom-end rot because symptoms are very similar, these diseases are actually
caused by two different fungi. Calyx end rot is a soft rot, which may expand to cover
about 1/3 of the end of a fruit, and is caused by Sclerotinia sclerotiorum. Dry-eye
rot is a shallow, hard rot over a smaller area, often with a red border, and is caused by Botrytis
cinerea. The diseases first appear about 1 month after petal fall, when the calyx end
of infected fruit reddens and then rots. (See Moldy Core which also causes fruit to redden
prematurely.) The diseases are most often seen on McIntosh, Paulared, and Delicious.
Spores are produced on either mummified fruit from previous years, or on a number of wild
host plants, and released from bloom through a few weeks after petal fall. The diseases
occur sporadically, and are associated with wet periods during bloom, petal fall and early
fruit set. While no controlled studies have been done on controlling these diseases, if a
benzimidazole fungicide is being used for other disease reasons (such as for scab
management) at petal fall and 10-14 days later, the application may help in preventing
infections in orchards where there has been an end rot problem, particularly if there was
a prolonged wet period during bloom-petal fall.
Fire Blight: Fire blight is
not a common disease in Vermont, but it can be very destructive on susceptible cultivars.
The threat of this disease is increasing in Vermont with the planting of new cultivars
that are more susceptible to the disease. The disease is caused by a bacterium, Erwinia
amylovora. The bacteria overwinters in bark tissues along the edges of cankers that
were produced from infections the previous season. Rain or insects can disseminate the
inoculum from the cankers to the stigmatic surfaces of blossoms where the bacteria
multiplies profusely. The bacteria can spread from blossom to blossom by bees or by rain.
The bacteria penetrates host tissue through wounds or natural openings in the presence of
water. Once inside the host, the pathogen will continue to multiply and kill plant cells.
Flowers, fruit, shoots, branches and trunks can become infected. Recently infected tissues
become water-soaked in appearance and may emit a watery, milky to light orange ooze on
humid days. As the tissue dies it turns from dark green to brown and black. When
susceptible cultivars are grown on susceptible rootstock, such as M.26 and M9, infections
of the scion or suckers at the base of the tree can spread into the rootstock and kill it
within a year. Successful management of fire blight takes an integrated approach including
resistant cultivars and rootstocks; removing sources of inoculum; attention to proper
nutrition so trees are not overly vigorous; and effective timing of treatment, when
warranted, to prevent infection.
Flyspeck and Sooty Blotch: These two diseases
blemish the fruit and present difficult disease management decisions because they do not
occur with the same intensity every year and they occur at a time when growers would like
to minimize pesticide applications. The diseases are caused by different fungi, and often
occur together on unprotected fruit. Sooty blotch is more easily controlled with
fungicides, and in most cases one or two summer fungicide applications provide adequate
protection. Flyspeck is more difficult to control. Growth of the pathogens depends on
humidity. If relative humidity is very high ( 95% or above) then the pathogens will grow.
Optimum temperatures for growth are at 60F to 75F, making warm summer nights ideal for
sooty blotch and flyspeck development. Many conditions can contribute to maintaining high
humidity in apple tree canopies, and therefore increase incidence of sooty blotch and
flyspeck. Practices which improve air circulation and drying will reduce the diseases.
Summer pruning, thinning to break up fruit clusters, mowing, and cutting dense hedgerows
are recommended where flyspeck and sooty blotch are problems. Cultural practices can
significantly reduce these disease problems, but fungicides are generally required to
maintain commercial fruit quality in wet, warm summers. Timing of fungicide applications
will depend on rain and the fungicides used.
Moldy Core: This disease is
characterized by a visible fungal growth in the seed cavity and core of some apple
cultivars, particularly Delicious and McIntosh. In storage, the disease may progress into
the flesh immediately surrounding the core causing a dry rot. While Alternaria spp.
are commonly isolated from moldy core apples, other fungi are also associated with the
disease. Sometimes, moldy core disease will cause fruit to redden prematurely during July
or August. More frequently, fruit do not show any external symptoms. Presumably, infection
occurs through the calyx when it remains open, allowing spores to enter the seed cavity.
Once the fungus is within the fruit, it is protected and fungicides have no effect. No
specific management tactic for this disease has been developed although fungicides applied
during bloom may provide some control.
Phytophthora: Crown, Root and Collar
Rots: These fungal diseases are caused by a number of
soil-inhabiting Phytophthora species including Phytophthora cactorum. Some
species may be ubiquitous soil inhabitants. The above ground symptoms of these diseases
are similar to those of vole-girdled trees, i.e. reduced vigor and growth, sparse foliage.
Fruit may be small and color prematurely. Collar rot is a disease that affects the lower
trunk (scion) above the rootstock-scion junction. Crown rot is a disease of the
underground rootstock portion of the trunk and base of primary roots Underneath the bark,
infected tissue is reddish brown.Root rot involves infection of the roots away from the
crown. Although less common, collar rot is more dangerous than crown or root rot since
with collar rot, the tree often declines or dies very quickly, especially if it is a small
tree. Occurrence of these diseases is sporadic and is much more likely where susceptible
rootstock is planted on a poorly drained site. Site selection and soil water management
are most important in preventing crown, collar, and root rots. Soils should have adequate
drainage throughout the year so that the soil is not saturated with water for prolonged
periods. Water saturation of soil for 24 hours is sufficient to initiate infections.
Selection of rootstocks adapted to the site is also important. Fungicides are important
but are not a substitute for good site preparation and the use of rootstocks adapted for
the intended orchard site.
Post-Harvest Rots: Several
fungi, notably Penicillium spp. (causeing blue mold) and Botrytis sp. (causeing
gray mold) are involved in post-harvest storage rots on apples. Generally, these rots are
not a problem unless apples become wet, as in the process of applying a scald inhibitor
(diphenylamine or DPA). Significant problems with Empire have occurred. The following
practices are part of an integrated management approach for post-harvest rots: (1)
Disinfect boxes and bins before they are reused; (2) Harvest fruit at the proper maturity
level. and handle fruit so as to minimize bumping and bruising; (3) During harvest, place
bins and boxes on sod and not on bare soil; (4) Cool fruit rapidly after harvest to
minimize opportunities for decays to become established; (5) Remove culls and leaf debris
from the packing shed daily. Post-harvest treatment of apples for control of storage rots
should not be done except when fruit must be treated for storage scald prevention or have
post-harvest calcium treatments.
Powdery Mildew: The powdery
mildew fungus, Podosphaera leucotricha, can infect foliage, shoots, blossoms and
fruit. Infected shoots have a slower growth rate; severely infected leaves are stunted and
never attain their normal size or shape. Infected leaves can become curled and brittle,
often dying prematurely. Photosynthesis is adversely affected and severe infections result
in reduced growth and vigor of the tree. Mildew can affect yield directly through the
abortion of severely infected blossoms or the russetting of infected fruit. Cultivars vary
in there susceptibility to the disease. Highly susceptible cultivars include Ginger Gold,
Cortland, Gala, Paulared, Idared, Rome Beauty, Granny Smith, and Jonathan. This disease
will increase in importance in Vermont as more susceptible cultivars are planted and as
warmer winters occur. The sterol inhibiting fungicides and the new strobilurin fungicides
are effective against powdery mildew but their repeated use raises resistance concerns.
Sulfur is an alternative mildew fungicide, but applications have to begin earlier (i.e.,
Tight Cluster) than the other fungicides and, because of short residual activity,
reapplied every 7 days for good results under high disease pressure. Benomyl and
thiophanate-methyl have been used against powdery mildew but it has been reported that
they have provided poor control in some orchards in recent years.
Rusts: Cedar-apple rust and
quince rust are not as important in Vermont as they are in southern New England but they
are still of economic concern. Cedar-apple rust is caused by the fungus Gymnosporangium
juniperi-virginianae, and is recognized by the brilliant yellow and orange leaf spots
and fruit lesions. Cultivar susceptibility varies. The quince rust fungus, Gymnosporangium
clavipes, does not cause leaf lesions, but fruit infections result in dark green
lesions near the calyx end which often distorts the fruit. Quince rust is more common on
Red Delicious. Quince rust is most likely to cause economic damage when trees stay wet for
more than 48 hours between tight cluster and late pink with an average temperature above
50 F. For susceptible cultivars, management of the rust diseases requires fungicide
applications. Spores are released from galls throughout any rainy period from around the
last week of April until mid-June, with peak releases from early pink to full bloom. In
orchards were rust diseases have been a problem, a fungicide effective against rust should
be applied before rains during this period. Removal of eastern red cedar and other juniper
trees surrounding the orchard aids in preventing rust infections.
Compiled by Lorraine P. Berkett, IPM Specialist, and M. Elena Garcia, Tree Fruit Specialist, Department of Plant & Soil Science, University of Vermont, Burlington, VT 05405. (802) 656-2630
Last modified: August 07, 2000
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