Botrytis fruit rot, or gray mold of caneberries—Botrytis cinerea
Botrytis fruit rot (gray mold) is a fungal disease of caneberries and numerous other hosts.
Identification
Flattened black masses of sclerotia (hardened masses of fungal mycelia) develop on the canes of Botrytis-infected caneberry plants in late winter. When canes are wet, the sclerotia germinate to form masses of gray mycelia (fungus vegetative growths) and spores (conidia). Opened flowers may become infected and the fungus will sporulate on them and kill the flowers. On infected drupelets, a watery rot may precede the development of grayish brown conidia and hyphae. The conidia and hyphae eventually cover the infected fruit. Infected berries left on the vines become mummified. If the weather is moist after harvest, the receptacles can be colonized by the fungus and sclerotia can develop there. In postharvest storage, white mycelia can grow to cover infected berries.
Life cycle
Botrytis fruit rot occurs under cool, wet conditions. The pathogen requires free water for the germination of sclerotia and spores and infection of the host. Physical damage to the plant increases disease incidence, especially during the rainy season.
Botrytis cinerea overwinters as sclerotia on infected canes and sometimes as mycelia in infected canes and leaves on the ground or lodged among canes. The main sources of infection are spores (conidia) from overwintering sclerotia, dead leaves, and mummified berries left on the canes or ground.
Spores are dispersed by rain, splashing overhead irrigation water, and wind. Flowers are not susceptible to infection until they have opened. Sporulation of the fungus and infections can occur on unpicked, overripe fruit left on the vine. Infections in berries generally remain dormant until fruit is nearly ripe or after harvest.
Solutions
Some red raspberry cultivars are partially resistant. Grow these where Botrytis fruit rot has been a problem.
Promote good air circulation to hasten the drying of plant tissue and allow berries to ripen in an open canopy. Create open canopies by heavily pruning to maintain a narrow row of plants, removing the first flush of primocanes (first-year, green, fleshy stalks), minimizing the application of nitrogen fertilizer, and controlling nearby weeds. Training systems for canes also help improve air movement and reduce development of gray mold. Growing caneberries inside macrotunnels (hoop houses covered with clear plastic) can greatly reduce the abundance of Botrytis and certain other diseases because the surface of plants is kept relatively dry.
Pick fruit during the morning when temperatures are cool, and promptly refrigerate the berries. Remove fruit mummies and dead leaves before tying up canes in the fall.
Gray mold is difficult to prevent with fungicides. If moist conditions are expected during bloom, before the fog or rain, a preventive fungicide spray can be applied. To potentially be effective the fungicide must thoroughly cover the plant. Potentially effective products include Bacillus subtilis, chlorothalonil, copper fungicides, horticultural oil, neem oil, and thiophanate methyl. If wet conditions persist, more than one application may be warranted. Note that some populations of Botrytis cinerea are resistant to certain fungicides. If more than one application is being made, rotate to a different fungicide.
Adapted from Pest Management Guidelines: Caneberries and Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide, University of California Statewide Integrated Pest Management Program (UC IPM).
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Black sclerotia of Botrytis fruit rot (center) on a gray, diseased section of a cane.
Blackberry fruit with grayish spores (center) of Botrytis fruit rot.
Spores of gray mold on an infected fruit.
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