Powdery mildew developed on lettuce in both research fields with this crop at LIHREC in Riverhead in 2013. Symptoms were first seen on 9 October. Both varieties being grown were affected: Romaine variety Ideal Cos and Boston variety Adriana.
The disease developed rapidly. Leaves became covered on both surfaces with characteristic white growth of powdery mildew. Symptoms developed on most leaves. Plants were full grown (at marketable size), which is when powdery mildew is reported as most likely to occur in lettuce.
This disease had not been observed on Long Island or in upstate NY during the past 25 – 40 years (personnel communication, T. Zitter, G. Abawai, and J Lorbeer). Conditions were unusually dry and warm during fall 2013. The typical heavy fall dews were rare. Powdery mildew develops on lettuce more commonly in Arizona when conditions are warm and dry.
Powdery mildew may have developed on lettuce in 2013 because conditions were atypically favorable for the pathogen and/or because a strain of the pathogen able to infect lettuce was present in 2013. This disease also occurred on lettuce in greenhouses and a winter high tunnel in several locations in the northeastern United States during winter 2013/14.
The pathogen causing powdery mildew on lettuce, Golovinomyces cichoracearum, also infects some weeds and cucurbit crops, but this fungus has very rarely been found to be the one causing powdery mildew on cucurbit crops on LI when examined. Microscopic examination of spores is needed to distinguish this fungus fromPodosphaera xanthii, the primary pathogen causing powdery mildew in cucurbits.