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SOUR ROT
Description
Symptoms
Disease Cycle
Control
Description:
Sour rot is a common disease of ripe grapes in the Southeast. The
disease can be very destructive if rainy periods occur just prior to
harvest.
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Symptoms:
Affected fruit become soft and watery, and fruit of light-skinned
varieties usually turn tan to light brown (Fig.
1). Masses of black, brown or
green spores may cover the surface of the fruit. Clusters with sour rot
often have a pungent vinegar odor.
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Disease Cycle:
The exact cause of sour rot is often impossible to determine. As
harvest time approaches, many different microorganisms, including
species of fungi, bacteria and yeasts, may attack grapes. Fungi
associated with sour rot include Aspergillus, Alternaria, Penicillium,
and Rhizopus. These fungi are naturally present on plant surfaces and
soil debris, and are spread by wind, rain, or insects to ripening fruit.
Ripening fruit begin rotting as soon as they are injured. Any type of
crack in the skin can allow entry of the sour rot organisms, whether
caused by birds, insects, hail, powdery mildew infections, or cracking
due to fruit swell following heavy rains. Tight clustered varieties are
particularly susceptible to sour rot. The vinegar-like smell is caused
by the production of acetic acid by Acetobacter bacteria, which are
carried by fruit flies and beetles to the clusters.
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Control:
Cultural - The best approach to control sour rot is to prevent
fruit injury by birds, insects, and other diseases such as powdery
mildew. Fruit damage due to growth-related causes can be prevented by
cultural methods, including fruit thinning and canopy management, while
closely monitoring irrigation and fertilizer use. If a rain period is
forecast, and fruit are mature or nearly mature, harvesting prior to the
rain will minimize fruit losses to sour rot.
Chemical - Chemical treatments are generally not effective in
preventing sour rot.
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