Gray Mold

Gray mold is one of the most destructive plant pathogens and attacks a wide variety of plants. Flower petals and ripening fruits and vegetables are particularly susceptible to infection, but leaves and stem tissues may also get infected. Young seedlings of several crops can be killed if infected with gray mold. In high relative humidity, the fungus may sporulate on infected tissues and produce masses of characteristic gray or brownish spores that become airborne, which is primarily how the fungus is disseminated. Spores must have moisture to germinate and infect.

Refrigeration at temperatures near 32°F will retard but not completely stop the development of gray mold; when infected tissue is warmed, decay can proceed rapidly.

Moisture often is more of a limiting factor than temperature. Free moisture is necessary for germination of Botrytis spores. Moisture is also necessary for growth within plant tissues, and low humidity may result in arrested growth of the fungus. However, growth can resume when moisture again becomes available.

Gray mold is most severe during times of the year when the humidity is high. In California, this is usually in the late fall and winter months, when rainfall is common. The worst time for disease development is from September to December, because there is an abundant amount of herbaceous vegetative material (crop refuse and dying summer plants) available for fungal colonization and, as a consequence, many spores are present in the air and on plant parts.

Botrytis cinerea produces innumerable asexual spores (conidia) that are moved by air currents. Because spores may readily develop in decaying vegetation and old flowers, elimination or reduction of sources of the spores is an important part of any control program.

• Remove old flowers before they become infected and function as spore sources. The fungus can develop and sporulate at low temperatures, so do not overlook old flowers and foliage in refrigerators.
• Avoid condensation of water on susceptible plant parts, as free moisture is necessary for germination and infection
• Avoid overhead watering during blooming. If this is the only method of irrigation available, irrigate early in the day so that the foliage can dry as rapidly as possible.
• Maximize the period between irrigations to further enhance drying of foliage and flowers.
• Increase plant spacing to increase ventilation and minimize leaf wetness. This can help reduce both disease incidence and severity.

Numerous fungicides are effective against B. cinerea but not all of them can be used on all crops. Some products can damage plants. To avoid damage and the development of fungal strains that are resistant to fungicides, growers should alternate different fungicides. The fungicides are preventives and must be applied before infection. In some crops, such as chrysanthemum, the lower foliage of crowded plants becomes infected and acts as a source of spores that then infect the flowers. In these crops, it is important to apply fungicides at an early stage when the lower foliage can be adequately covered by the chemical.