Bunch rots—Botrytis cinerea and other factors
Symptoms of bunch rot often appear first on old flower parts and spread to fruit and foliage.
Leaves, flowers, and green or ripe fruit on or near the soil in the dampest or densest areas of the canopy turn light brown and develop a gray or brown fuzzy growth of fungal spores. Occasionally stems may be affected. Fruit or plants shrivel and rot and often develop flattened, hard, black masses, called sclerotia, under rotted parts.
Following injury, berries are susceptible to invasion by many microorganisms that frequently enter through insect- or bird-feeding sites, mechanical cracks, or lesions. The Botrytis fungus that causes bunch rot survives in decaying plant material in or on the soil and as sclerotia. It usually infects only flowers and senescing or injured tissue, especially plant parts in contact with damp soil. However, green, healthy tissue may be affected. Grapes are infected at the flower stage, but the fungus remains dormant until fruit mature; bunch rot then develops if conditions become cool and damp.
Use planting densities and training techniques that provide maximum air movement through the plant canopy. Avoid excessive nitrogen fertilizer that will encourage excess growth. Trim leaves around bunches soon after bloom to improve air circulation. Remove diseased plants as soon as they appear, and pick ripened grapes as soon as possible. Harvest fruit in advance of rainy or humid conditions when possible. Fungicides are available but should not be needed if leaves are pruned and no rains occur during bloom. Chemicals are not recommended for summer bunch rot.
Early spring infection on leaf
Late season infection
Initial fruit infection