By Mike Matheron, UA Extension Plant Pathologist

As we move into the late stages of the lettuce production season in the desert southwest, the appearance of Botrytis gray mold may become more evident. The appearance of fuzzy gray growth at the base of maturing lettuce plants is a sign that the fungusBotrytis cinerea is present.

The gray growth contains vast numbers of spores dispersed in the air. This fungus can survive in the field as sclerotia in the soil, as a pathogen on many different crop and weed plants, and on crop debris.

When cool temperatures and high humidity prevail, spores landing on senescent or damaged lettuce tissue will germinate and then grow into healthy plant leaf and stem tissue which can lead to plant collapse and death.

Lettuce plants can be predisposed to infection by environmental factors including frost or heat plus by the activity of other plant pathogens, including Bremia lactucae (cause of downy mildew),Rhizoctonia solani (cause of bottom rot), and the Sclerotinia species that cause lettuce drop.

Botrytis and Sclerotinia are related fungal pathogens. Fungicides effective against one are usually active against the other. Fungicide applications are most effective when plants are young and gray mold is not yet present. The efficacy of later applications to older plants is not known.

Contact Matheron: (928) 726-6856 or matheron@ag.arizona.edu.