Strategies to Control Powdery Mildew: Part I

These days finds us trying to familiarize ourselves with the strategies available to control the mildews that we will surely encounter once the growing season is upon us. We have already had our encounter with powdery mildew (PM) last growing season and I blogged about it in Erysiphe necator: Powdery Mildew, which shows what infected grapevines look like. I found an article online written by Wayne Wilcox in the March, April 2003 issue of Practical Winery and Vineyard Journal called Strategies to Control Powdery Mildew and read it with interest. Wilcox writes that although cleistothecia do require at least a brief rain to release their ascospores, neither these spores nor the conidia require moisture in order to germinate and cause infection. The primary factors that control the spread of PM are temperature and relative humidity. The part of the PM lifecycle that becomes relevant in the spring is shown by the red circle in the figure below:

The ascospores from the overwintering chleistothecia (chasmothecia), can be released with as little as 0.1 inch of rain when temperatures are 50 ^oF. The released ascospores are the primary infection which develop into lesions that produce another kind of spore called the conidia. The conidia forms the powdery or dusty appearance that is visible to the eye and is the cause of the secondary infection that can result in the exponential growth of the disease through the vineyard when the temperature is in the ideal range between 68-77 ^oF.

Crop Susceptibility:
• Leaves are highly susceptible to infection while they are expanding, but become resistant soon after they’re fully expanded
• Berries are highly susceptible from bloom until shortly after fruit set, but become much more resistant afterwards

Since powdery mildew is a disease of young, juvenile tissues, sprays applied from the start of bloom through bunch closure are responsible for the lion’s share of PM control on the berries.
Temperatures above 90 ^oF as well as rain or condensation on the leaves and fruit are detrimental to disease development.
Stay tuned for the next blog, Strategies to Control Powdery Mildew: Part 2 for what kinds of steps can be taken to control powdery mildew.

References:
1. Wayne Wilcox, Strategies to Control Powdery Mildew, Practical Winery and Vineyard Journal, March, April 2003.
2. The illustration is from the Compendium of Grape Diseases, 1988, American Phytopathological Society, St. Paul, MN. USA, pg. 10.