A qPCR Assay for the Quantification of the Overwintering Chasmothecia of Erysiphe necator in Grapevine Bark

Powdery mildew (PM) disease causes serious losses in Mediterranean vineyards, where suitable environmental conditions promote conidial infections. The frequency and intensity of these infections are directly linked to the amount of primary inoculum, i.e., the chasmothecia embedded in the trunk. In this study, we set up a protocol to extract and quantify chasmothecia in grapevine bark samples based on a quantitative polymerase chain reaction (qPCR) assay. Moreover, we observed PM severity and ascocarp production on leaves in the first season and primary infection in the following season in different grapevine cvs. with known PM susceptibility levels. The qPCR analysis showed a significant relationship between DNA concentration in bark samples and primary infection (R = 0.970) and disease severity development (R = 0.776), as well as chasmothecia development on leaves (R = 0.455). The results demonstrate that this methodology can be used for quantifying chasmothecia, improving current methods based on visual counting, proving the interrelationships between PM epidemics and chasmothecia, as well as refining PM disease prediction models and subsequent fungicide application. Rapid and easy quantification of ascosporic inoculum will greatly facilitate the reconciliation of control actions to the risks posed by greatly differing levels of ascosporic inoculum.