Wayne Wilcox: GRAPES 101 Powdery Mildew: How Important is Overwintering Inoculum?

Sometimes, it is just really nice to come across an article that is so timely. We have been advised by our vineyard consultant, Fritz Westover that we should be spraying our vineyard to beat down the powdery and downy mildew inoculum that our grapevines unfortunately contracted during this growing season. The current issue of The Appellation Cornell Newsletter has an article written by Wayne Wilcox called GRAPES 101 Powdery Mildew: How Important is Overwintering Inoculum?¹
What we are trying to do currently is to eliminate the sexual spores called ascospores that overwinter in a fruiting bodies called cleistothecia. The studies conducted by Wayne Wilcox and his colleague David Gadoury show that when the spray regiment was continued until Labor Day, the disease severity the following year was 5-fold less.

If we do not take these steps now we risk the health of our vines next spring. This is because if ascospores in the cleistothecia survive the winter, they are released the following spring during rain events and dispersed by the wind. If the vine is at a stage between budbreak and bloom, each ascospore that lands on green tissue can cause an infection that can result in the production of thousands of asexual spores called conidia when temperatures are in the 60's and 70's Fahrenheit (longer at cooler temperatures). Powdery mildew fungus can produce a new generation in as few as five days, allowing for it to develop explosively once it gets started if control is less than perfect.¹
So we are spraying, hoping to avoid this horror show next spring.
References:
1. Tim Martinson and Wayne Wilcox, GRAPES 101 Powdery Mildew: How Important is Overwintering Inoculum?, Appellation Cornell Newsletter, Issue 15 September, 2013.
2. The illustration is from the Compendium of Grape Diseases, 1988, American Phytopathological Society, St. Paul, MN. USA, pg. 10.