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Gray Mold

Gray mold (Botrytis cinerea) causes raspberry fruit to rot and become moldy while still on the canes. The gray mold fungus can also infect blossoms, stems and senescent leaves of raspberry plants. Gray mold is most common in fall-bearing raspberries, in dense patches of raspberries, and in patches with little air movement due to surrounding trees and buildings. In ripe fruit, gray mold may not appear until after picking, resulting in reduced shelf life. The disease can rapidly spread among fruit in a container.

Identification

gray-mold

Raspberries with gray mold.

Michelle Grabowski, UMN Extension

Mature red fruit are most commonly infected with gray mold, although blossom blight occasionally occurs in cool, wet weather. Velvety gray spores completely cover infected areas of the fruit. The top of the fruit is often infected first, near where the fruit attaches to the stem, but this very quickly spreads to the entire fruit. Infected fruit remain attached to the plant and dry up into a shriveled black mummy. The shriveled fruit may produce new spores in wet weather.

Lower leaves on densely planted canes are commonly infected. Mature leaves turn dark brown in color and fall off. Young leaves are unaffected. On green primocanes, gray mold causes pale brown lesions that often have darker growth rings layered within the infection. Lesions on canes often start as ovals but grow to encircle the cane and spread both upward and downward. As the cane matures and turns brown these lesions disappear. Over winter, the epidermis turns white in the infected area and eventually falls off in spring, revealing hard round to oval shiny black fungal sclerotia pressed against the stem. Lateral branches that develop from buds within gray mold infected canes grow poorly or do not develop at all.

Important biology

Gray mold overwinters on infected canes, decaying leaves and other plant matter. During humid, cool (59-72°F) weather, fungal spores are produced and blown by wind to susceptible blossoms or leaves. Plants that remain for more than one day will have many infections. The majority of fruit infections start as flower infections. The fungal spores germinate and grow through the stigma into the developing fruit. The damage usually does not show up until the fruit ripens several weeks later. At first, the infected fruit is soft, with a water-soaked appearance. After a few days, infected parts of the fruit become covered in velvety gray fungal spores that can spread from the infected fruit to other ripe fruit.

Only mature to senescent leaves can be infected with the gray mold fungus. The fungus moves from the leaves into the canes and spreads both up and down the cane. Buds within the infected area may be killed or may grow poorly the following year. Sclerotia, hard black fungal resting structures, are produced on infected canes and allow the fungus to survive from one season to the next.

Management

The most effective way to reduce gray mold in raspberries is to reduce free water on leaves and blossoms. Choose a location with full sun and good air movement. Raspberries should be planted in narrow rows that dry quickly after dew, irrigation or rain. Use drip irrigation or soaker hose to keep leaves and blossoms dry. If sprinkler irrigation is the only option, water in the morning on a sunny day so plants dry quickly. Always remove dead floricanes to increase air movement within the row and thin new primocanes to maintain a narrow bed size.

Remove potential overwintering sites of the fungal pathogen by removing dead floricanes and pruning out any canes showing symptoms of gray mold on the cane. Infected canes should be composted well away from the raspberry patch in a compost pile that heats up. Alternatively, infected canes can be burned, buried, or brought to municipal composting facilities depending on local waste management regulations.

Gray mold pressure can be reduced by regularly harvesting the fruit and not allowing overripe and rotten berries to accumulate in the patch. Pick raspberries early in the day when fruit is still ripe. Take care not to bruise or damage fruit during harvest. Cool raspberries quickly after picking. Remove infected fruit from the patch to reduce spread of the fungal pathogen to developing blossoms and fruit.

If the patch has a history of gray mold, and there is a long wet spell during bloom, then fungicides are advisable. The best time to apply fungicides to prevent gray mold is during bloom, when the first infections can start. Fungicides can also be sprayed during picking, but they are less effective at this time. Several formulations of captan are labeled for gray mold in raspberries. Captan will prevent gray mold from infecting fruit but will not cure gray mold that has already started.


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