From 1973, when we first planted grapes, until 1996, downy mildew was almost unheard of in Maryland. Then, for one season, we became a rain forest. It not only rained every day, it would start to rain right after spraying while the tractor was headed for the barn. After the growing season, we heard pretty much the same story from growers all over the eastern U.S.
Under these conditions, systemic spray material does not have a chance to enter the plant before it is washed away. For powdery mildew, we had sulfur. For downy mildew we had Mancozeb which may not be applied to grapes within 66 days of harvest. Bordeaux mixture (a copper and hydrated lime preparation) would have helped, but, having no prior experience with Downy Mildew, we were too dumb to buy it.
Chardonnay, although highly susceptible to powdery, showed some resistance. Seyval (SV 5-276) was not impacted. Cabernet Franc was not impacted. The Cabernet Sauvignon was devastated.
By mid August, 6 weeks prior to normal harvest, the Cabernet vines were almost defoliated. The fruit ceased to ripen. But the worst was yet to come.
At pruning time in the spring of 1997, the vines looked great. There were multiple choices of canes and spurs. Cross sections of buds showed bright green. Everything looked upbeat. Then, as the normal time for bud swell and bud break came and went, very little had happened. The buds that did break had little vigor and set small wimpy clusters. Throughout the season, 10 and 12 bud canes that looked great at pruning time showed no sign of life.
Again, growers in the region compared notes and consulted the experts. The verdict was that the buds had not developed properly due to early loss of foliage. The buds that did break were secondary buds and the crops were reduced accordingly in both quantity and quality.
Be aware of the susceptibility of your vine varieties to downy mildew. In rainy seasons, alternate applications of spray material specific to downy with the contact killer, Bordeaux mixture. Use a surfactant to help spread the spray material on leaf and berry surfaces. Keep in mind that if your varieties are susceptible and downy is not controlled, you are facing two bad seasons instead of one.
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