Factors that Influence Disease Onset from Overwintering Powdery Mildew Mycelium in Grapevine Buds and Establishment of Protocols for Risk Assessment and Control

Powdery mildew, caused by Uncinula necator, can overwinter in the dormant buds of grapevines and become a source of primary inoculum in the spring. Grapevine dormant buds develop during the previous season to become the current seasons fruiting canes. Developing dormant buds were collected periodically following the onset of shoot growth in the spring of 1998. The buds were sectioned and examined microscopically for the presence hyphae and infection. In 1997 and 1998, a field site in Madera County, California was observed for infected buds resulting from perennated U. necator. Early season treatments were tested for the reduction of bud infections over time and the number of infections were counted and mapped. Leaf haustoria of U. necator were observed to be ovoid in shape, ranging in size from 3.75 x 7.5 u.m to 7.5 x 12.5 urn with a mean size of 6.86 x 8.56 iim. These sizes are consistent with those previously measured. Mycelia and haustoria were found in the buds from the earliest collection date, approximately three weeks after shoot growth. The size and shape of haustoria were consistent with the size range and shape of haustoria in the inoculated leaves, and the hyphae averaged 3.76|um, which was consistent with previous observations. Specific tissues observed to carry infections in the buds were trichomes and prophylls. Infected buds on each vine in each 1/3 acre block were mapped. The early control procedures yielded a reduction in the incidence of bud infections over time in the treatment blocks. A repeated measures ANOVA using the year as the within subject, showed a significant difference in the treatment blocks from 1997 to 1998. The number of infections per vine and the total number of infections per block significantly decreased. The control blocks, however, increased slightly overall but there was no significant difference between years.