IMPACT OF POWDERY MILDEW (LEVEILLULA TAURICA) ON YIELD AND FRUIT QUALITY OF PROCESSING TOMATOES IN CALIFORNIA

Powdery mildew (Leveillula taurica) became more severe on tomatoes in California’s Central Valley during the 2007 through 2013 seasons. Eighteen uniform field trials evaluating chemical control programs were conducted over a four-year period to identify economical and effective control programs, but also to better understand the impact of powdery mildew on yield and fruit quality in processing tomatoes. Of the three trial years, disease pressure was highest in 2009 and 2012; lighter in two intervening years. Disease severity and foliar necrosis at harvest were always significantly reduced by the fungicide programs. Mildew reduced fruit yield by 52% in one trial; in all other trials there was no yield effect. Soluble solids were increased by an average of 0.6 °Brix in sulfur-treated plots relative to non-treated plots; the greatest increase was 1.4 °Brix in one trial. Disease increasing only one month prior to harvest may affect soluble solids without affecting yield, while earlier high disease pressure may significantly reduce yields.