The greenhouse environment provides ideal growing conditions for both plants and plant pathogens. The high relative humidity and lack of air circulation often found beneath the plant canopy are especially ideal for the pathogen Botrytis cinerea, commonly called gray mold. This pathogen infects many greenhouse ornamentals and is considered the second most important plant pathogen in the world.
Common disease symptoms include leaf spots, blight and stem cankers with the fuzzy gray mold being a key way to identify the Botrytis culprit. Flowers are especially prone to infection by the gray mold and when the infected petals fall to the foliage, leaf infections result. Botrytis spores move by air and are carried to healthy plants where new infections begin. Infection requires water, which is needed for spore germination. Penetration of the plant by Botrytis can be direct or indirect through natural openings or wounds. Small leaf spot symptoms may quickly combine into large blighted areas under high relative humidity and wet conditions. Botrytis usually becomes established and produces spores on older leaves near the moist soil surface and under the plant canopy and flowers. Dead tissue in the plant pot or on the greenhouse bench or floor can support gray mold and spore production.
There are times in the spring when Michigan growers face a perfect storm for a Botrytis outbreak. Elements of the storm include overcast, chilly, wet weather, and maturing floriculture crops with flowers.
Preventing and controlling gray mold relies on a multi-pronged approach including cultural and chemical controls. Sanitation reduces the spore load by removing dead and dying plants, leaves and flowers from greenhouse production areas. Improving air flow to reduce relative humidity is also important. Relative humidity can also be reduced by slightly increasing the temperature via heating. Limit watering whenever possible and water at a time of day when the foliage can dry quickly. While these strategies are helpful, they are often not enough to eliminate gray mold during Michigan’s spring damp weather complete with overcast skies and fungicides may be needed.
Read more at Michigan State University (Mary Hausbeck and Blair Harlan)