Gray Mold (Botrytis Blight)

Botrytis blight or “gray mold” is a widely distributed disease caused by the fungus Botrytis cinerea. 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.

Symptoms

Gray mold (Botrytis cinerea) on harvested strawberries.
Edward Sikora, Auburn University, Bugwood.org

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 or raspberries 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. On some plants, such as roses, it can cause slightly sunken areas called cankers on the stems. Corms and bulbs of perennials and annuals may rot when infected with gray mold.

Plants Commonly Affected

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, impatiens, 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, plums, raspberries, strawberries, blackberries, blueberries, and grapes. In the vegetable garden, look for gray mold on tomatoes and beans.

Prevention & Treatment

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 easily infected plant parts for gray mold to become established.

Keep Leaves Dry: Avoid overhead watering and wetting of plants since this fungus is easily spread by splashing water and wind.

Provide Good Air Circulation: Do not overcrowd plants. Use a wide spacing between plants to promote drying. Gray mold thrives in shaded and crowded plantings and in areas of poor air circulation. Plant in sites with adequate sunlight.

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 fungicide product to determine if it will control gray mold, if it is labeled for use on the specific plants, and how often the plants should be sprayed. For fruit and vegetable crops, the label will state the pre-harvest interval (the time that must elapse between spraying and harvesting). The label will also tell the spray interval required before reapplying sprays.

Annual and perennial bedding plants, flowering and foliage plants, and seedlings in beds, flats, or pots may be sprayed with fungicides containing chlorothalonil, thiophanate-methyl, copper fungicides, or neem oil (clarified hydrophobic extract). However, neem oil is a rather weak fungicide. 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 fungicide product for when and how often it should be used 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 for 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 on most vegetable crops for gray mold control. Check the product label for the pre-harvest interval.

Gray mold on grapes, blueberries, peaches, nectarines, and plums can be controlled with sprays of captan. Brambles (blackberries, raspberries, etc.), strawberries, and cherries can be sprayed with either captan or copper fungicides for gray mold control.

Always check fungicide product labels for the plants that can be sprayed and the rate of application. It is always best to spray in the late evening to reduce the impact on pollinating insects. See Table 1 for a listing of brands and products for gray mold control.

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.

Table 1. Fungicides to Control Gray Mold

Pesticides are updated annually. Last updates were done on 9/24 by N. Jordan Franklin.

Originally published 05/99