Gray Mold (March 30, 2016)

The appearance of a fuzzy grayish growth on lettuce or onions usually signals the presence of Botrytis cinerea, the fungus responsible for gray mold. In the desert southwest, gray mold can be a concern for lettuce and onion growers, in addition to anyone who enjoys eating strawberries, as this fungus can rapidly turn them into furry-looking inedible fruit. The plant pathogen flourishes under conditions of high humidity. On onion and lettuce crops, profuse amounts of spores, invisible without magnification, are produced on the gray growth and dispersed in the air. When favorable temperature and humidity levels exist, spores landing on senescing or damaged lettuce leaves or onion tissue will germinate and grow into healthy plant leaf and stem tissue, leading to potential plant collapse and death. Disease symptoms on lettuce are similar to those caused by Sclerotinia minor and S. sclerotiorum, the causal agents of lettuce drop. Actually, Botrytis and >Sclerotinia are closely related fungal pathogens; however, the visible mycelial growth of Sclerotinia on plants is white. Fungicides effective against Sclerotinia are usually active against Botrytis as well. As with Sclerotinia, fungicide applications for Botrytis management are beneficial when plants are treated before and not after the appearance of fungal growth and disease symptoms.