Prepared by Nancy Doubrava, HGIC Information Specialist, and James H. Blake, Extension Plant Pathologist, Clemson University. (New 05/99.)
Botrytis blight or "gray mold" is a widely distributed disease caused by the fungus Botrytis species. It can infect some vegetables, soft fruits, flowers, trees and shrubs, especially when conditions are cool and damp. The fungus usually occurs on plant debris or weak plant tissue, such as old flowers, leaves and overripe fruit. It can be very destructive, since it can spread quickly to rot healthy plant tissue.
Gray mold can cause different symptoms on different kinds of plants. Typically, as its name suggests, gray mold causes a gray, fuzzy coating on aging flower blossoms and soft, ripe fruits. A cloud of grayish-white spores may be noticed when infected leaves or flowers are picked.
Infection usually begins as brown to gray circular spots that later become fuzzy when the fungus produces gray masses of spores. Ripe strawberries left too long in the refrigerator often develop gray mold on the surface of the fruit. The disease can cause spotting and decay of flowers, leaves, fruits and berries. In some plants such as roses, it can cause slightly sunken areas called cankers on the stems. Corms and bulbs may rot when infected with gray mold.
Gray mold affects a wide range of annual and perennial plants. Flowers with thick succulent petals, such as begonias, peonies and geraniums, are particularly susceptible. The disease also commonly affects African violet, amaryllis, calendula, camellia, bulbous iris, delphinium, dahlias, larkspur, snapdragon and hyacinth.
Many fruits, vegetables, and berries are also easily infected by gray mold, especially after being harvested and moved to cool storage areas. Commonly infected are apples, pears, peaches, raspberries, strawberries, blackberries, raspberries, blueberries and grapes. In the vegetable garden, look for gray mold on tomatoes and beans.
Cultural Controls: Remember, cool, damp weather favors the development and spread of this disease. Gray mold is not difficult to control using the following cultural methods.
Sanitation: Following good sanitation practices is one of the best ways to reduce this disease. Collect and discard faded flower blossoms and fallen petals. In the vegetable garden, remove infected plants immediately after harvest. Plant tissues that are stressed, aging or inactive are great hosts for gray mold to establish itself.
Keep Leaves Dry: Avoid overhead watering and syringing of plants, since this fungus is easily spread by splashing water and wind.
Provide Good Air Circulation: Do not overcrowd plants. Use wide spacings of plants to promote drying. Gray mold thrives in crowded plantings and in areas of poor air circulation.
Maintain Healthy Plants: Follow recommended cultural practices, especially proper fertilization, irrigation and pruning practices.
Chemical Controls: Chemical control of gray mold using fungicides is rarely needed on most plants. Fungicides can be applied on a protective basis before disease develops, especially during periods of high humidity and cool temperatures. Several fungicides are approved for homeowner use to control gray mold on specific vegetables and flowers. Always check the label of the chemical to determine what is recommended for specific plants.
Annual and perennial bedding plants, flowering and foliage plants, and seedlings in beds, flats or pots may be sprayed with fungicides containing thiophanate-methyl, copper sulfate pentahydrate or neem oil (clarified hydrophobic extract). Caution is recommended, however, especially with the new bedding plants developed in the past 10 years. It is best to test the fungicide on a few plants first before treating all of them. Check the label on the chemical to determine what is recommended for specific plants.
Snap and green beans infected by gray mold can be sprayed with fungicides containing chlorothalonil. In problem areas, start spraying at early bloom and continue to apply once a week. Wait a minimum of seven days between the last fungicide application and harvest.
Tomato plants infected by the gray mold fungus have light tan or gray spots that are covered with a brown mold on the upper leaf surface. Fungicides for the home garden that contain chlorothalonil can be used in problem areas.
Postharvest rots can be a problem for many fruits and vegetables in the home garden. Mix 1 tablespoon of fresh bleach (sodium hypochlorite 5.25%) in one gallon of water. Dip fruit into the bleach solution, rinse in clean water and dry fruit. Change the bleach solution frequently when it gets dirty.
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This information is supplied with the understanding that no discrimination is intended and no endorsement by the Clemson University Cooperative Extension Service is implied. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.