Lettuce powdery mildew

By Mike Matheron, UA Extension Plant Pathologist

As we enter the final stretch of this lettuce production season in the desert southwest, powdery mildew can be an economic concern in some lettuce fields. Powdery mildew, caused by the fungus Golovinomyces cichoracearum, is primarily a disease of lettuce plants at or nearing maturity.

The disease is first observed as very small spots of white fungal growth on upper and lower leaf surfaces of the oldest leaves. From these initial infection sites, the fungus releases numerous spores which are carried in the air, which upon landing on lettuce leaves initiate additional infections under favorable temperature and moisture conditions.
The most favorable temperature range for spore germination is 65 to 77 degrees F. Relative humidity at or above 85 percent is required for infection, growth, and sporulation by the pathogen. Low light intensity also favors powdery mildew development.

These requirements are often all met for one to several hours daily especially on lettuce leaves near or at the soil surface in a maturing lettuce planting. As little as four days are needed from infection to production of a new crop of pathogen spores.

Depending on environmental conditions and the particular susceptibility of the lettuce variety, preventative applications of a fungicide may be needed to prevent economic loss to the crop.

The oldest leaves where initial powdery mildew infections develop will not be harvested. However, the leaves serve as nurseries and launching sites for spore production and release which can lead to infection of the marketable portion of the lettuce plant.

In recent field trials, fungicides that provided excellent control of powdery mildew on lettuce included Microthiol Disperss (wettable sulfur), Procure (triflumizole), Quintec (quinoxyfen), and Rally (myclobutanil).

Initiating fungicide treatments before or at the very latest at the first sign of infection on the oldest leaves will result in the best disease control.

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