Lettuce Powdery Mildew (February 3, 2016)

February is a month when powdery mildew can make its initial appearance in maturing lettuce plantings. The disease, caused by the fungus Golovinomyces cichoracearum, is first observed as very small spots of white fungal growth on both upper and lower leaf surfaces of the oldest leaves. From these initial infection sites, the fungus continues to grow on the leaf surface and release vast quantities of spores which are carried in the air, and 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 °F. Relative humidity at or above 85% is required for infection, growth and sporulation by the pathogen; however, free moisture will actually kill powdery mildew spores. Low light intensity also favors powdery mildew development. These requirements are often all met for several hours daily, especially on lettuce leaves near or at the soil surface in a maturing lettuce planting. As little as 4 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 being grown, preventative applications of a fungicide may be needed to prevent economic loss to the crop. The oldest leaves containing the first powdery mildew infection sites will not be harvested; however, these leaves serve as nurseries for production and release of spores, which can infect the marketable portion of the lettuce plant. In recent field trials conducted at the Yuma Agricultural Center, fungicides that provided excellent control of powdery mildew on lettuce included Fontelis (penthiopyrad), Merivon (fluxapyroxad+pyraclostrobin), Microthiol Disperss (wettable sulfur), Procure (triflumizole), Quintec (quinoxyfen), and Rally (myclobutanil). Initiating fungicide treatments before or at the very latest at the very first sign of infection on the oldest leaves will result in the best levels of disease control.