Evaluation of Fungicides for Management of Powdery Mildew on Lettuce in 2007

Powdery mildew, caused by the fungus Golovinomyces cichoracearum (formerly known as Erysiphe cichoracearum), can develop rapidly in spring lettuce during March and April in western Arizona, as the crop nears maturity, when moderate to warm temperatures and dry environmental conditions prevail. The first signs of disease can occur as early as December or January. Successful chemical control of powdery mildew requires the presence of an effective fungicide on plants before disease onset, followed by successive applications of materials to maintain disease control until harvest. A field trial was conducted in 2007 to test and compare the efficacy of some registered as well as new fungicides, applied alone or in a rotational treatment program, for management of powdery mildew. Foliar applications of treatments were made Jan 24, Feb 1, Feb 16 and Feb 28, 2007. Among treatments, the degree of powdery mildew control ranged from virtually complete to minimal; however, all treatments significantly reduced disease severity compared to untreated plants. Treatments that reduced the severity of powdery mildew more than 90% compared to untreated plants included Procure alternated with Quintec, Quintec, Microthiol Disperss, and Procure alternated with Microthiol Disperss. Yield loss due to rejected lettuce heads usually would begin to occur when the powdery mildew disease rating exceeds 2.0. Fungicide treatments that kept disease severity below this level (in addition to the treatments already listed) include V-10118, Procure alternated with Quadris, Procure, Switch, Forum + Cabrio, Endorse, Maneb + Reason, and Endura. This trial was initiated as a combined downy and powdery mildew trial; therefore, some of the products were placed in the test specifically for downy mildew. Due to low humidity levels and no rainfall during the trial, no downy mildew developed; however, some of these downy mildew fungicides, such as Forum, Maneb, and Reason, significantly suppressed powdery mildew. Phytotoxicity symptoms were not noted on lettuce for any of the materials tested