Black rot can affect all new growth – leaves, petioles, shoots, tendrils and berries – but it is most destructive to fruit clusters. Fruit infections occur from bloom until the berries become naturally resistant (about 3 to 5 weeks after bloom in most varieties). The first symptom, a whitish dot within a rapidly expanding brown area, appears 10 to 14 days after infection. Within a few days, the berry starts to shrivel and becomes a hard, blue-black mummy.
If berries are infected close to the onset of natural resistance, lesions remain localized. The fungus over-winters in mummies within the vine or on the ground. Ascospores are released shortly after bud break until about 2 weeks after bloom and are dispersed by wind and rain. Infected tissues can also yield conidia, which are dispersed by rain splash and cause secondary infections. The optimum temperature for disease development is 80°F (27°C). At this temperature, the wetness period required for infection is only 6 hours (see table below).
Number of wetting hours required for black rot infection at various temperatures.
Ave. temp. (F) / Hr. of leaf wetness
50 / 24
55 / 12
60 / 9
65 / 8
70 / 7
75 / 7
80 / 6
85 / 9
90 / 12
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