Gray Mold of Strawberry
Click for larger image Gray mold (Botrytis cinerea) on strawberries (Fragaria)

Gray mold of strawberries is caused by a fungus, Botrytis cinerea, which infects both the flowers and fruits. Because of this, Botrytis can greatly reduce fruit yields and is considered one of the most damaging diseases of strawberry. Botrytis is most prevalent during prolonged cool, wet weather during bloom and near harvest.

Symptoms and Diagnosis

Blossoms commonly turn brown and die. A soft, light brown rot may appear on any part of the berry, but generally occurs first in the area of the cap, destroying the berry within 48 hours. The infected fruit spot is at first a light brown color and somewhat soft in texture. As the entire berry becomes infected, the rotted area becomes firm and turns a darker brown color. Fruits soon "mummify" and, like the blossoms, become covered with a gray, dusty powder which are the spores of the Botrytis fungus. Berries resting on damp soil or touching infected plant parts are most commonly infected.

Life Cycle

Botrytis fungi overwinter as dark-colored, resting bodies (sclerotia) on dead tissue. In the spring during cool humid weather, spores form and spread by wind or water to wounded or extremely soft plant tissues. Infection can also occur from growth of fungal mycelium from previously infected plant parts. The fungus can survive on decaying vegetation so it can infect healthy plants throughout the growing season.

Integrated Pest Management Strategies

1. Remove infected plant parts. Collecting and removing infected plant parts can slow the spread of the disease. This should be done frequently especially during fruit bearing time.

2. Improve air circulation around the plants. Space plants widely and prune leaves so that adequate air flow may speed drying of the vegetation. Work with plants when they are dry.

3. Avoid spring applications of nitrogen fertilizer. High nitrogen levels promote excessive leaf growth and available surfaces for infection.

4. Harvest regularly. Remove and dispose of rotten or severely damaged fruit throughout the season.

5. Move plants to a better location.Select sites for planting that have good air circulation, are not shaded, and not subject to frost injury.

6. Use fungicide sprays, if necessary. Apply a fungicide at 5–10% bloom and at full bloom. Fungicides may be reapplied every 7–10 days during wet seasons. Pesticides registered for use include captanchlorothalonil (Daconil), copper, iprodione (Chipco), mancozeb, sulfur, thiram, and ziram.

Organic Strategies

Strategies 1, 2, 3, 4, and 5 are strictly organic approaches. Of the fungicides listed in Strategy 6, consult the Organic Materials Review Institute (OMRI™) for appropriate organic copper products.

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