THE WEATHERVINE - Weather change will give grapevine fungus the boot


The June weather review the state climatologist recently issued confirmed a few facts most of us who pay even the slightest attention to such matters already knew: Washington experienced one of its coolest and wettest Junes on record.

For many locations, including Walla Walla, it was the wettest June since weather records have been kept. The 31/2 inches of rain recorded at the airport last month was 300 percent of normal; no June since record-keeping began here in 1949 has exceeded that figure.

In addition, the average June temperature of 63.5 degrees ranks as the eighth coolest of the sixty-three on record.

This cooler and wetter regime had some serious implications for local viticulturists.

Like other fruiting plants, grapevines are subject to a host of diseases which, if left unaddressed, can seriously diminish the quantity and quality of the fruit. Among these, the fungus known as powdery mildew is by far and away the most widespread and troublesome. Growers worldwide spend millions of dollars annually on various fungicides and their application to combat this scourge.

As with many fungal plant pathogens, the weather plays an important role in determining their severity. Most, if not all, have an ideal range of temperature and humidity conditions within which they flourish.

In the case of powdery mildew, an environment that offers moderate to high humidity and temperatures between 70 and 85 degrees Fahrenheit provides the optimum microclimate for the causative fungus, Erysiphe necator, to grow and reproduce on any green grape tissue, including leaves, shoots and unripe berries.

On the other hand, hot weather (in excess of 95 degrees) and exposure to ultraviolet light are hostile to the disease - both of which have large implications for the management of grapevine canopies as relates to inhibiting its inception and spread.

In a cool, damp spring and early summer such as Walla Walla and the rest of the state experienced last year as well as the month just ended, grape growers were obliged to be extra diligent with their powdery mildew treatment plans and spend two or three times more on fungicides than normal owing to the prolonged period of near-ideal weather conditions for the disease. Some fungicides like sulfur and stylet oil will wash off in a heavy rain. Others that are systemic in nature will retain their efficacy, but the disease pressure exerted by the optimum environment for its growth will require shortening the interval between spray applications.

To create a less favorable environment for powdery mildew, growers take steps to open the grape canopy to allow more light and air to penetrate the interior by pulling leaves and positioning shoots or removing those that are unfruitful, thus reducing the humidity, providing more UV light and allowing for better spray penetration.

Better yet, the hot summer weather that finally arrived in the past few days will discourage the growth of the fungus.

The change is due to the clockwise circulation around a sprawling dome of high pressure centered over the Four Corners of the desert Southwest, which has been pumping hot air northward . Along with that heat, some monsoonal moisture from Mexico has been entrained in the southerly flow.

The combination of the heat and moisture was responsible for touching off a few scattered thunderstorms this past weekend, mostly over the higher terrain.

For those preferring something cooler, the high pressure should relax sufficiently by Wednesday to allow for some modest cooling for the latter part of the week.

A lifelong fan of weather and the Baltimore Orioles, Jeff Popick is an instructor at the Enology and Viticulture Center at Walla Walla Community College and manages the school's teaching vineyard. Send your questions and comments to him at


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