The Distribution of Early Season Grapevine Shoots Infected by Uncinula necator from Year to Year: A Case Study in Two California Vineyards

¹ BASF Corporation, 26 Davis Drive, P.O. Box 13528, Research Triangle Park, NC 27709
² Dept. of Plant Pathology, University of California, Davis, CA 95616-8749.

ypemah{at}basf-corp.com

Uncinula necator (Schw.) Burr., the causal agent of powdery mildew on grapevines (Vitis vinifera L.), survives the winter in the San Joaquin Valley in California predominantly by perennation in buds. Infected shoots arising from these buds (flag shoots) are important inoculum sources early in the growing season. Studies were conducted in two San Joaquin Valley vineyard sites over a three-year period to determine whether infected shoots tended to occur on the same vines from year to year. The studies also had the goal to determine whether the occurrence of infected shoots was influenced by the location of spurs retained from shoots infected the previous year. The number of vines with infected shoots in two consecutive years was greater than expected for a random infection process. The incidence of infected shoots arising from spurs retained from shoots infected the previous year was significantly greater than expected for a random shoot infection process. However, most infected shoots arose on spurs that were not infected the previous year, and they were not clustered around spurs retained from shoots infected the previous year. Removal of spurs retained from shoots infected the previous season during dormancy, or removal of flag shoots early in the season as a control measure by itself will probably not sufficiently reduce flag shoots incidence the following years

Key words: grape, Vitis vinifera, grape powdery mildew, Uncinula necator

Submitted on April 1, 1999
Revised on September 13, 1999