Botrytis Blight or Gray Mold

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 multicelled structures called sclerotia and (2) single-celled, thick, dark-walled chlamydospores. The fungus can persist in the greenhouse for long periods in either structure in the absence of plants.

Infection

Botrytis is a weak pathogen that 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 overfertilization, 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 combination of fungicides. However, Botrytis can develop resistance to certain chemicals.
  
  

Chemicals listed below in the row have the same way of killing the fungus (same mode of action):

COMMON NAME	TRADE NAME
azoxystrobin	Heritage
chlorothalonil	Exotherm Termil, Daconil, PathGuard
copper	Phyton 27, Nu-Cop, Kocide
fenhexamid	Decree
fludioxonil	Medallion
iprodione	Chipco 26019
mancozeb	Dithane, Protect T/O, Pentathlon
mancozeb + copper	Junction
myclobutanil	Systhane
Trichoderma harzianum	PlantShield, Trichodex (biological controls)
Streptomyces griseoviridis	Mycostop (a biological control)

Botrytis develops resistance to some fungicides when used exclusively over a long period of time. Therefore, do not rely entirely on one chemical or a group of chemicals that act similarly (see chart above). It has been found that it is best to mix chemicals (if mixing is allowable as stated on the label) that act differently. It has been found that reduced-rate mixtures of two or more chemicals provide as good protection as full-rate mixtures.

Resistance to fungicides containing benzimidazole (Topsin M, Benlate*, SysTec*, Domain FL*, Cleary's 3336*, Zyban*, Duosan*) is common in greenhouses. These fungicides are not likely to control Botrytis. Resistance to Ornalin* and Chipco 26019* has been found in several facilities. Do not rely entirely upon these chemicals for control. If you suspect they are not controling Botrytis in your greenhouse, cease using them.

*Trade name