Botrytis Or Gray Mold

Gary W. Moorman, Professor of Plant Pathology

The plant pathogenic fungus Botrytis is found virtually everywhere plants are grown. It is fast growing, can grow on many different sources of nutrients, survives well in the greenhouse, and can attack many different types of plants. The disease caused by Botrytis is commonly called Botrytis blight or gray mold.

The fungus:
Botrytis at first appears as a white growth on the plant but very soon darkens to a gray color. Smoky-gray "dusty" spores form and are spread by the wind or in water. In greenhouses, any activity will result in a release of spores. Even automated trickle irrigation systems, when turned on, trigger a release of spores. These spores are often found on the outside of seeds. The spores can remain dormant on plant surfaces as long as the life of the plant in some cases. Botrytis forms two types of resting structures on or in infected plant tissue: 1) very dark brown or black multi-celled structures called sclerotia and 2) single-celled, thick, dark walled chlamydospores.

Infection:
Botrytis must have nutrients or some food source before it invades the plant. Nutrients leaking from wounded plant parts or from dying tissue such as old flower petals provide the required nutrients. From this food base, the fungus becomes more aggressive and invades healthy tissue. A dark to light brown rot forms in the diseased tissue. High humidity conditions favor the growth of this fungus.

Sites Of First Infection

• Wounded tissue such as large stubs left after taking cuttings.
• Fading flowers.
• Leaves on which fading infected flowers have fallen.
• Broken stems or injured leaves.
• Leaves damaged by over-fertilization, spray damage, or mechanical injury.
• Seedlings grown under cool, moist conditions.
• Cuttings taken from plants with heavy infestations of Botrytis.

Management

• Sanitation is the first important step. Remove dead or dying tissue from the plants and from the soil surface. Remove this refuse from the greenhouse. Do not throw debris under benches or on walks. Sanitation alone is not sufficient to control this fungus. The fungus can produce 60,000 or more spores on a piece of plant tissue the size of your small finger nail. Even one spore can infect a plant and cause disease.
• Avoid injuring plants in any way. Do not leave large stubs of tissue on stock plants when taking cuttings.
• Heat and ventilate greenhouses to prevent high humidity conditions . This may only require extra venting early in the day when moisture has condensed and before sunlight has warmed the air. Even lowering the humidity slightly can have a significant effect on Botrytis. Outdoor planting should be planned to provide good air circulation patterns. This is the most important means of inhibiting Botrytis activity.
• Added protection is available for many crops by applying a fungicide or biological control agent. Note that some Botrytis populations are resistant to certain chemicals. This becomes a problem when those fungicides are used exclusively over a long period of time. Therefore, do not rely entirely on one chemical or a group of chemicals that act similarly. It has been found that it is best to mix chemicals (if mixing is allowable as stated on the label) that act differently. See the chart below. Each can be used at half the label rate. It has been found that reduced rate mixtures of 2 or more chemicals provides as good protection as full rate mixtures and, in some cases, may be less phytotoxic.

Resistance to fungicides containing benzimidazole (FRAC Group 1, see below) is common in greenhouses. These fungicides are unlikely to control Botrytis and are not recommended for that reason. Resistance dicarboximide fungicides (FRAC Group 2) has been found in about 50% of greenhouse operations. Do not rely entirely upon these chemicals for control.

Active Ingredients and Trade Names of the Chemicals

Fungicides and Fungicide Resistance Management - Certain fungicides, usually systemic fungicides, are said to be 'at risk' to the development of resistance if they are used repeatedly. See the Risk Level in the above table (1 = low risk; 3 = high risk). The Fungicide Resistance Action Committee has developed a numbering system in which chemicals with the same FRAC Group number have the same mode of action (See http://www.frac.info/frac/index.htm ). It is recommended that chemicals at high risk be used sparingly and in rotation or mixed with chemicals with different modes of actions (different FRAC number).

Biological Controls