The risk assessment index in grape powdery mildew control decisions and the effect of temperature and humidity on conidial germination of Erysiphe necator

Powdery mildew (Erysiphe necator) is a major disease of grapevines (Vitis vinifera) in Chile. Severe outbreaks have occurred recently despite the use of strict fungicide programs to control it. The objectives of this study were to evaluate the infection risk assessment index (RAI), to predict conditions for E. necator infection, and to study the effect of temperature (T), relative humidity (RH) and free moisture (FM) on conidial germination and disease development. Conidial germination was affected by T, RH, and FM. There were significant (p < 0.001) interactions between E. necator isolates and T and between isolates and RH. Conidial germination was optimal at 25 degrees C. There was no germination at 5 degrees C and 35 degrees C. At 20 degrees C, conidia germinated at a low RH (33-35%). Germination increased at a RH between 47 and 90% but decreased at higher RHs. Powdery mildew development on Carmenere,Chardonnay,and Merlot vines increased linearly from 6 degrees C to 23 degrees C. These grape cultivars were all equally susceptible to E. necator. Incubation periods varied. It was 13 to 14 d at 20 degrees C or 23 degrees C, 19 to 24 d at 10 degrees C, and more than 23 d at 6 degrees C. Grape powdery mildew was markedly decreased when inoculated leaves were wet at 1 or 72 h post inoculation. In conclusion, RAI, determined on the basis of air T, was useful to decide on fungicide applications between grape bud burst and veraison in Chile.