Interruption and Reduction of Erysiphe necator Chasmothecia Development Utilizing Fungicidal Oil

Grape powdery mildew epidemics, caused by Erysiphe necator, are initiated by the release and subsequent germination of ascospores from mature chasmothecia. Interrupting the development of chasmothecia prior to overwintering may reduce or prevent the overwintering of E. necator in vineyards, thereby reducing initial disease incidence observed the following growing season. At the end of the grape growing season in 2014 and 2015, one application of Organic JMS Stylet Oil (Vero Beach, FL) at a rate of 10 ml/liter was applied on four treatment dates using an air-assisted backpack sprayer onto Chardonnay grapevines within a vineyard. Leaves were collected chasmothecia enumerated from the first observation of chasmothecia primordia development until the onset of rains (approximately 5 weeks). In 2014, all stylet oil treatment plots developed significantly fewer chasmothecia than nontreated control plots in 2014 (P = 0.04), and there were no differences in treatment date observed. No treatment differences were observed in 2015. Because chasmothecia were still produced despite stylet oil treatment, a single post-veraison stylet oil application to interrupt chasmothecia development would not be an economical management option for reduction of grape powdery mildew.