Many fungi are capable of rotting mature or near-mature fruits of strawberry, grape, raspberry, blueberry and blackberry. Serious losses can occur under favorable environmental conditions for disease development. One of the most serious and common fruit rot diseases is gray mold. The gray mold fungus can affect petals, flower stalks (pedicels), fruit caps and fruit. In wet, warm seasons, there is probably no other disease capable of causing a greater loss of flowers and fruit. The disease is most severe during years with prolonged rainy and cloudy periods during bloom or during harvest. Although berries of all grape varieties are susceptible to bunch rot, losses generally are greater on tight-clustered varieties of Vitis vinifera and French hybrids.

Symptoms:

Young blossoms are usually very susceptible to infection. One or several blossoms in a cluster may show blasting (browning and drying) that may extend down the pedicel. Fruit infections usually appear as soft, light brown areas on the fruit which enlarge rapidly. Infected fruits remaining on the plant usually dry up, "mummify," and become covered with gray, dusty spores, which give the disease its name "gray mold." Fruit infection is most severe in well-protected areas of the plant, where the humidity is high and air movement is poor. On strawberry, berries resting on soil or touching another decayed berry or a dead leaf in dense foliage are most commonly affected. The disease may develop on young green fruits, but fruits become more susceptible as they mature. The disease is not usually detected until fruits are mature at harvest time. After picking, mature fruits are extremely susceptible to gray mold, especially if bruised. The handling of infected fruit while picking will spread the fungus to healthy ones. Under favorable conditions for disease development, healthy berries may become a rotted mass within 48 hours of picking.

Early (left) and late (right) stages of botrytis fruit rot on strawberry. Photos courtesy of the Ohio State University Extension.

Causal Organism:

The gray mold fungus is capable of infecting a great number of different plants. The disease cycle is very similar for strawberries, grapes, blueberries and brambles. The fungus overwinters as minute, black fungus bodies (sclerotia) or as mycelium in plant debris, such as dead strawberry or raspberry leaves. Recent research has shown that nearly all of the overwintering inoculum in strawberry plantings comes from mycelium in dead strawberry leaves within the row or planting. In early spring, the mycelium becomes active and produces large numbers of microscopic spores (conidia) on the surface of old plant (leaf) debris in the row. Spores are spread by wind throughout the planting where they are deposited on blossoms and fruits. They germinate when a film of moisture is present and infection can occur within a few hours.

Temperatures between 70 and 80 degrees F and free moisture on the foliage from rain, dew, fog or irrigation water are ideal conditions for disease development. The disease can develop at lower temperatures if foliage remains wet for long periods. Grapes and berries are susceptible to Botrytis during bloom and again as fruits ripen. Recent research indicates that most fruit infection actually occurs during bloom; however, symptoms usually do not develop until close to harvest. During bloom, the fungus colonizes healthy or senescing flower parts, often turning the blossoms brown. These blossom infections establish the fungus within the receptacle of the young fruit as a "latent" or "quiescent" infection. The fungus generally remains latent in developing (green) fruit until the fruit starts to mature (the fruit sugar content increases and the acid content decreases to a level that supports fungus growth), at which time the fungus becomes active and symptoms (rot) appear. Thus, the most critical period for applying fungicides to control gray mold is during bloom. This is an important point to remember when considering fungicide applications for controlling this disease.

Control:

• Select a planting site with good soil drainage and air circulation. Plants should be exposed to direct sunlight. Plant rows with the direction of the prevailing wind to promote faster drying of foliage and fruit.

• A good layer of straw mulch (or other material) between the rows or around the plants aids greatly in controlling fruit rots. The mulch acts as a barrier that reduces fruit contact with the soil.

• Proper spacing of plants and timing of fertilizer applications are also important. Excessive applications of nitrogen fertilizer, especially in the spring before harvest, can produce excessive amounts of dense foliage. Shading of fruit by thick foliage prevents rapid drying of the fruit during wet periods and creates ideal conditions for disease development.

• Good weed control is very important. Weeds prevent air movement in the plant canopy. This slows drying time of flowers and fruits and increases the chances for infection. Pick fruit frequently and early in the day as soon as plants are dry. Cull out all diseased fruit but do not leave them in the field. Handle fruit with care to avoid bruising. Refrigerate promptly at 32 to 50 degrees F to inhibit the gray mold fungus.

• Fungicides are an important disease management tool in commercial plantings, but are generally not effective unless they are timed properly and used in conjunction with the above cultural practices. Homeowners are encouraged to emphasize the use of cultural practices in order to avoid the use of fungicides.

Adapted from Michael A. Ellis, Ohio State University Extension, 1999