I. Introduction: Brown rot is one of the most important diseases of stone fruits in the mid-Atlantic
region. Field losses of nectarines can be extensive if conditions favorable for disease
development occur during the blossoming or preharvest and harvest periods. Losses of peach
vary with susceptibility of the cultivar.
II. Symptoms: Typical disease symptoms induced by M.
fructicola include blossom and twig blight (photo 2-49), cankers (photo 2-50), and a
fruit rot (photo 2-51). The fungus often produces conidia profusely on sporodochia on
infected areas. The first indication of the disease in the spring is the rapid death of
blossoms which, as they turn brown, often become affixed to the twig in a gummy mass,
later becoming covered with a grayish to tan spore mass. Frequently, following
colonization of the blossom, the fungus enters the shoot where it causes a canker on which
spores are also produced. Shoot blight symptoms will occur if the fungus girdles the
shoot. Leaves on such shoots turn tan to brown and may remain attached for several weeks.
Cankers formed following blossom or fruit infection appear as brownish, sunken areas, that are often covered
with gum. These cankers support the formation of conidia in wet weather and harbor the
fungus over the winter. Usually, the tree is able to restrict cankers to small oval areas
at the junction of the shoot and the infected blossom or fruit. Cankers and killed shoots
may be colonized by other aggressive canker-causing fungi such as Leucostoma spp.
Brown rot on ripening or mature fruit typically develops as
a rapidly spreading brown necrosis. Under optimum conditions for the fungus, entire fruit
may be rotted within 48 hours of infection. The infection produces a soft dry rot,
although occasionally the skin remains firm. On nectarines, brown rot sometimes occurs as
quiescent infections which can be detected as small, circular, necrotic lesions on
immature fruits. As fruits mature, decay spreads from the lesion throughout the entire
fruit. Immature or mature fruit with brown rot infections will sporulate profusely,
shrivel, and become tough grayish-black mummies (photo 2-52). These mummies may drop to
the ground, where apothecia (photo 2-53) may develop, or remain attached to the tree
through the winter. Decaying fruit in cold storage or transit may appear black with little
or no sporulation.
III. Disease Cycle: M. fructicola overwinters in orchards as
mycelium on mummies, fruit stems, blighted blossoms and twigs, and cankers. Sporodochia
develop under cool, wet conditions during the winter and early spring. Occasionally,
cup-like apothecia of M. fructicola which produce ascospores can be found on fruit
mummies under the tree, but they are not usually common in mid-Atlantic commercial
orchards. In years when apothecia were common, severe blossom blight was noted in peach
and apricot orchards, but severe blossom blight also can occur in the absence of
apothecia. Generally, conidia from mummies and cankers on stone fruit trees and other
sources (for example, flowering ornamental plants of plum or quince, or wild plantings of
plum) are believed to be the primary inoculum sources.
Conidia of M. fructicola are generally formed during late
spring when temperatures range from 55 to 77 F (13-25 C). Conidia are disseminated by wind
and rain and germinate rapidly under favorable conditions. Optimum temperatures for
blossom infection of peach range from 72 to 77 F (22-25 C). Between 32 to 86 F (0-30 C),
temperatures above or below the optimum range delay germination but do not inhibit it.
Inoculum concentration also interacts with temperature and wetness duration to influence
incubation period and disease incidence and severity. Under optimum temperature
conditions, fruit infections can occur with only three hours of wetness when inoculum
levels are high. Longer wet periods during infection result in shorter incubation periods
so symptoms develop more rapidly. Large amounts of inoculum with highly favorable
environment produces a high potential for heavy losses.
blossom blight can be severe enough to reduce the crop, early sporulation on even a few
infected blossoms provides more inoculum for later fruit infections. The subsequent
invasion of shoots also enables the pathogen to survive in the host for long periods. In
some areas, infections of flowers may result in active or quiescent infections that either
cause decay of green fruit or become active prior to harvest. Quiescent infections of
peach and nectarine have not been reported in the eastern U.S., perhaps because blossom
infection is less common here than in other locations.
Sporodochia of M. fructicola on infected blossoms
and shoots may produce viable conidia throughout the remainder of the growing season,
although sporulation from infected blossoms tends to decline over the summer. These
conidia may infect injured green fruits; nonabscised, aborted fruits; green fruits thinned
after the pit hardening stage and dropped under the trees; and ripening fruits as they
mature. For most stone fruits, susceptibility of fruit to infection increases as fruit
color begins to develop. Infection may occur by direct penetration of the germinating
spore through the cuticle or lenticels in the fruit. In most orchard situations, inoculum
produced on early maturing cultivars fuels a continuing outbreak that affects late
Insects (nitidulid beetles and honey bees) also can be
important as vectors of the fungus during fruit ripening, carrying conidia to injury sites
produced by oriental fruit moth, Japanese beetle, green June beetle, and other insects
that injure fruit. Wounded fruit are infected much more readily than nonwounded fruit. At
harvest, apparently healthy fruit usually are contaminated with spores which, under
favorable conditions, may later decay during storage and marketing.
IV. Monitoring: During or after pruning (before the
pink stage), monitor a minimum of 20 sample trees per block for the presence of fruit
mummies (photo 2-52) and cankers (photo 2-50). A total of one to ten mummies and/or
cankers, and more than ten mummies and/or cankers represents levels of moderate and high
risk, respectively, for blossom infection under the appropriate environmental conditions.
Before bloom, monitor the orchard floor under sample trees
for the presence of apothecia of the brown rot fungus. These are more likely to occur in
the wettest areas of the orchard on mummies partially buried in soil and/or among weeds
(photo 2-53). Finding any apothecia represents a potential high risk for blossom
infection. Remove cankers (photos 2-49, 2-50) surgically if possible or prune out the
entire diseased area. Monitoring for and removal of cankers is best done at the same time.
At shuck fall, examine ten shoots on
each sample tree for the presence of blossom infection (photo 2-49). A total of one to ten
blossom infections and greater than ten blossom infections represents moderate and high
risk, respectively, for fruit infection during the preharvest and harvest periods.
Fruit susceptibility to brown rot (photo 2-51) increases
rapidly as fruit begin to color. Monitor ten fruit on each sample tree for disease
incidence. Greater than two infected fruit per ten acres (eight trees sampled) represents
a high risk for a brown rot outbreak at this time. Monitor approximately every three to
five days during the preharvest period. Insect, bird and hail damage to ripening fruit can
result in wounds which can be quickly colonized by the rot fungus. Brown rot will show up
first in areas near sources of inoculum and where fruit may be physically injured. An
estimate of the potential for postharvest rot to develop can be determined by harvesting
10 mature, healthy fruit from each sample tree. Wound each of these fruits several times
by stabbing them with a knife [1/4 inch (6-7 mm) deep] and holding them in a closed
plastic bag at room temperature for 24 to 48 hours. Record the percent rotted fruit.
Brown rot may develop during storage and shipment if fruit
are not handled properly during and after harvest. Monitor daily for developing decay in
packed fruit being temporarily stored by checking fruit throughout a minimum of
Cultural practices. Sanitation is
essential if your orchard is to be considered a low risk for a brown rot epidemic. The
practices listed below, if followed, should minimize brown rot spore populations and limit
the likelihood of an epidemic when conditions are favorable for rapid disease development.
(1) Remove all remaining fruit from the
tree after the final picking. This practice limits infection of fruit peduncles and twigs
thus reducing the number of brown rot cankers. In addition, this practice prevents the
situation where overwintered mummies within the tree would be immediately adjacent to
susceptible blossoms in the spring. Furthermore, removal of remaining fruit after final
picking separates the practice of removing mummies from spring pruning. Where these
practices are separated, the grower has more latitude to selectively prune (following a
severe winter with high bud mortality, for example) without increased risk of blossom
(2) Fruit thinning practices influence the
carry over of brown rot during the summer months and into the fruit ripening season. In
general, fruit thinned before pit hardening decompose rapidly; whereas, fruit thinned
after pit hardening become infected on the orchard floor and serve as spore sources for
the disease. Although it would be ideal to thin all cultivars before pit hardening, this
is not practical because: (i) early, mid-season and late cultivars reach the pit hardening
stage at the same time; and (ii) thinning early season cultivars at pit hardening or
before favors formation of split pits. If thinning is done after pit hardening, the
thinned fruit should be removed from the orchard (thin with picking baskets or rake and
shovel the litter). Alternatively, direct a sprayer nozzle downward so that thinned fruit
receive some fungicide.
(3) In spring, monitor for blossom
infection and prune out any cankers and infected shoots.
(4) In spring, just prior to and during the
blossom period, examine the orchard floor for apothecia. Their presence requires that
blossoms be thoroughly protected with fungicide sprays during wet periods.
(5) Prune to avoid excessive overcrowding
of branches to increase air circulation, promote rapid drying, and increase light and
(6) Fertilize to maintain optimum
(7) Avoid dumping rotten fruit in one
location, which could become the starting point for disease and insect outbreaks in the
(8) Pick and handle fruit carefully to
avoid injuries; remove field heat from the fruit promptly after harvest by hydrocooling or
forced air cooling; use clean containers; keep packing areas clean.
Chemical management: Fungicides
are recommended generally in a protective program for a complex of diseases, including
brown rot, scab, and powdery mildew. Fungicides are to be applied prior to fungal
infection that occurs during rain periods. Blossom infections are controlled with two or
three fungicide sprays during the bloom period, with the number of sprays often varying
from year to year depending upon the weather, the susceptibility of the stone fruit
species, the length of the bloom period and the type of fungicide. Sweet cherry is
generally more susceptible than sour cherry, and nectarines are more susceptible than
peaches. The preharvest period for stone fruits is especially susceptible to brown
rot infection. Begin preharvest sprays when the fruit begin to color, about 3 weeks
before harvest. Controlling insects, especially those that directly injure fruit,
will help prevent infection. Some postharvest treatments include hot water
treatments and chlorine dips.
Chemical control -
Chemical control - home
orchardists (pdf file - Acrobat Reader required)
Text prepared by A. R. Biggs, K. D. Hickey,
and K. S. Yoder
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