Gray mold, caused by the wide-spread fungus Botrytis cinerea, affects most vegetable crops, as well as many ornamental and cannabis plants in greenhouses and indoor production facilities. In this article, we provide information to help you identify and manage the disease.
Symptoms and signs of the disease
In general, gray mold symptoms include brown discoloration, water soaking and dense velvety or fuzzy whitish gray mold (mycelium and spores) growing on the surface of infected areas (Fig. 1). Botrytis spores appear abundantly on dead or dying tissue (Fig. 2). Botrytis spp. can act as a secondary decay organism and can be difficult to distinguish if Botrytis came first in your crop.
Seedlings can become infected when the stem is in contact with the soil or substrate and result in pre or post damping off, delayed development or collapse. When starting seedlings, make sure to avoid over irrigating during cooler parts of the day.
Leaves will often develop V-shape blight lesions. Stems will turn white and develop cankers, reduce girth and wilt. Flowers and fruit will turn brown and soft and the gray spores will be abundant and clearly visible.
Penn State Extension reported that Botrytis “… can produce 60,000 or more spores on a piece of plant tissue the size of your small fingernail. Even one spore can infect a plant and cause disease.”
What type of spores are we talking about? This fungal pathogen is made up of one celled spores called conidia that form on branched conidiophores (Fig. 1), which cleverly lends Botrytis its name since “Botrys” translates to “a bunch of grapes” in Greek. With a hand lens or dissecting microscope, you can look for the characteristic grape-like cluster arrangement of spores (Fig. 3)
Start healthy. Inspect incoming cuttings, tissue and even purchased seeds. Shipping boxes are a perfect environment for gray mold (high humidity inside the box and moderate temperatures of 65 – 75°F in the shipping truck) and young, succulent tissue is particularly vulnerable.
Stay on top of sanitation. Remove dead or dying tissue from plants, substrate surface, benches or any other hard surfaces. Be careful to spread spores. Remove the infected tissue from the greenhouse by bagging it before moving around the greenhouse.
Avoid damaging plants and apply proper pruning methods. Conidia usually do not penetrate living tissue directly, but rather infect through wounds, or by first colonizing dead tissues (old flower petals, dying foliage, etc.). Therefore, avoid wounding plants and fruit during growth. When you prune your vegetable plants, try to break the petioles close to the stem and prune early on sunny days so that any wounds have time to dry before the night.
Increase ventilation. Proper ventilation will reduce leaf wetness and increase air circulation, as Botrytis persists better in high humidity conditions. Humidity can be reduced by exhausting the moist air and replacing it with outside air that is cooler and drier. For more information to reduce greenhouse humidity, click here.
Rotate fungicides. Fungicides are another layer of control. Penn State Extension has reported fungicide resistance for isolates of Botrytis, so fungicides applications should be used as a coordination with other cultural practices.