Gray Mold (April 2, 2014)

The disease gray mold does occur on lettuce and onion crops in the desert southwest. Botrytis cinerea, the fungus that causes this disease, produces an easily recognizable grayish fuzzy growth on affected plants when humidity levels are high. Although invisible without magnification, profuse amounts of spores are produced by the gray growth and dispersed in the air. When favorable temperature and humidity levels exist, spores landing on senescent or damaged lettuce or onion tissue will germinate and grow into healthy plant leaf and stem tissue, leading to potential plant collapse and death. This outcome on lettuce is similar to that caused by Sclerotinia minor and S. sclerotiorum, the causal agents of lettuce drop. Botrytis and Sclerotinia are related fungal pathogens; however, the visible mycelial growth of Sclerotinia on plants is white. Fungicides effective against Sclerotinia are usually active against Botrytis. As with Sclerotinia, fungicide applications for Botrytis management are beneficial when plants are treated before the appearance of fungal growth and disease symptoms. Due to scarcity of rainfall and periods of high humidity, gray mold occurrence has been limited during this vegetable production season.

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To contact Mike Matheron go to: matheron@ag.arizona.edu.