Powdery mildew (Uncinula necator) has been a consistent disease problem in California vineyards for over a century. For nearly that long, elemental sulfur has been applied for its control. Agricultural bulletins dating to the 1890s describe the treatment schedule still in use today: regular applications of sulfur at 7- to 14-day intervals during the period of vine susceptibility. The interval may be adjusted for local conditions and grape variety, but no systematic method for determining the best timing has been previously presented. The past recommendations have led to under- or over-use of sulfur in years when environmental conditions are more or less favorable for epidemic spread.

This work was funded through the Smith-Lever Act grant, VC-IPM. The authors thank the 58 vineyard managers and their personnel who participated in research plots and field trials. Also, the authors are grateful for the assistance of UC Cooperative Extension farm advisors: James Kissler, Rudy Neja, Philip Phillips, John Foott, Harry Andris, Dick Bethel, Keith Bowers, and Don Luvisi.