Growing & Winemaking


How the mildew model conserves fuel, materials

February 2009
by John Kiger, Kiger Family Vineyards
Kiger Family Vineyards
John Kiger checks the gauges of the mildew tracking station at Kiger Family Vineyards.

During the past couple of seasons I have been transitioning from conventional to organic farming practices. In this transition I anticipated a significant increase in the costs of spraying for powdery mildew control. Organic controls for powdery mildew typically require spraying every 7-14 days, as opposed to every 14-21 days using conventional fungicides. Although organic control programs tend to use less expensive fungicide materials, labor, equipment and diesel costs increase significantly with more frequent spraying.

Typical mildew management begins shortly after budbreak in late March or early April and continues at pre-defined intervals until ripening of the grapes begins in mid- to late July. Ideally, the interval between sprays is defined by a combination of two factors: the mildew risk index and the fungicides used, with each fungicide product specifying a minimum interval corresponding to periods of high mildew risk and a maximum interval for periods of low risk.

In practice, most growers don't have access to accurate mildew risk index information and are not able to fine-tune their spray programs to their specific vineyard conditions. I decided that I could both lower my costs and reduce the amount of fungicide materials I apply each season by installing a weather station in my vineyard that would calculate the powdery mildew risk index according to specific weather conditions at my site.

I found a cost-effective solution to be the WeatherTracker Pest Alert Model #330 from Spectrum Technologies. The WeatherTracker calculates and updates the Gubler-Thomas Powdery Mildew Risk Index daily based on the specific weather conditions in my vineyard. Using the WeatherTracker and the risk index calculations, I have been able to eliminate two or three sprays during each of the past two seasons. The WeatherTracker cost me less than $500 and easily paid for itself in the first season.

Each spring I spray JMS Stylet Oil® shortly after bud break, when I have three to five fully open leaves on most vines, and reset the WeatherTracker Powdery Mildew Risk Index to zero for the new season. As a powdery mildew eradicant, the Stylet oil spray helps kill any overwintering powdery mildew in the vineyard for a clean start to the new season. My next spray does not occur until the risk index is triggered for the new season, which takes three consecutive days, each with six or more consecutive hours with temperatures between 70° and 85°F. That typically occurs anywhere from three to five weeks later and is often just prior to bloom.

As soon as the index is triggered, I do another Stylet oil spray and begin tracking the index daily as the guiding factor for my spray intervals for the remainder of the season. I believe it is prudent to use an eradicant, such as Stylet oil, in the first spray after the index triggers to eliminate any mildew colonies that may have begun forming. I then use the model to schedule protective fungicide applications according to the application rates and intervals specified on the product label.

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