Istituto di Patologia Vegetale, Università degli Studi di Milano, via Celoria, 2, 20133 Milan, Italy
ARSIA via Pietra Piana, 30, 50121 Florence, Italy
Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva, NY 14456
Density and viability of populations of cleistothecia of Uncinula necator from bark, leaves, and soil were determined in three vineyards in the Florence and Siena provinces of Tuscany for 3 years. A higher density of cleistothecia was found on fallen leaves than on bark. However, the percentage of viable cleistothecia was higher on bark. No viable cleistothecia were recovered from soil. U. necator overwintered as mycelium in dormant infected buds, which gave rise to flag shoots, only in Santa Cristina, where 20 and 92 flag shoots per hectare were detected before bloom in 1994 and 1995, respectively. Disease incidence and severity increased similarly at Corti, Fornace, and at Santa Cristina, although powdery mildew epidemics started from ascospores only in Corti and Fornace, whereas flag shoots were present at Santa Cristina. Cleistothecia were formed in autumn in both 1994 and 1995, and their dispersal started in late September to mid-October, with the maximum number of cleistothecia trapped in funnels during the second half of October. Cleistothecia appear to function as the sole source of primary inoculum for grape powdery mildew in some Italian vineyards and serve as additional sources of inoculum where the pathogen also overwinters in infected buds. In Australia but not in New York, the pathogen also overwinters as cleistothecia on fallen leaves.