Powdery mildew has met a very few crops it did not like to infest. Producers of crops like grapes, pears, strawberries, cherries and cucurbits often struggle to determine when is the ideal time to initiate control measures since the best time to control it is before it appears.

How long before is the question?

University of California, Davis plant pathologist Doug Gubler and others developed a grape risk assessment model to aid in the decision-making process that also turned out to be a good indicator for powdery mildew control in melons, squash, cucumbers and other cucurbit crops.

Linked to a computerized network of field weather stations, the powdery mildew risk assessment index for grapes gives advanced alerts of the temperature-sensitive disease. Gubler has evaluated the temperature-based powdery mildew risk index with cucurbits for the past three years. “We just took it, as is, from the grape index. It seems to work really well for cucurbits.”

This risk assessment index is available from the UC IPM Web site and through other commercial companies.

“Powdery mildew,” Gubler says, “is serious on cucurbits virtually anytime on the coast and mostly during periods of cool, humid conditions in the Central Valley. It prefers temperatures somewhere from 70 to 85 degrees.

“A period of two to three weeks of temperatures in that range for six hours a day is all it needs to run out of control.”

Moves in fast

Crops in the Central Valley pick up the disease in mid- to late-summer, and then as temperatures cool, it becomes rampant to cause severe damage. “It moves in very fast on melons on the West Side of the San Joaquin Valley once a good canopy has developed and there's plenty of moisture beneath.

“It goes first to the petioles and undersides of leaves, just where some people don't look carefully. Before a grower knows it's there, he can start seeing the plants collapse. Then it is too late.”

And that's why planning ahead for when to use a fungicide is the first order of business, he says. “Scout for it as the canopy develops and particularly when temperatures are in the operative range. You have to turn some leaves over. “It can appear anywhere on pumpkins and squash, but on melons and some varieties of cucumbers generally it stays on the lower sides of leaves and the petioles.”

Using the risk assessment index can help growers determine when to apply products to control powdery mildew before it gets a toehold. And, fortunately there are an increasing number of products to use. One of the newest is Procure 50WS, labeled late last year for use on cucurbits by the California Department of Pesticide Regulation.

Gubler says, “Procure has worked extremely well in our trials with cucurbits since 1998.”

Procure is a broad-spectrum, demethylation inhibitor (DMI) fungicide manufactured by Crompton/Uniroyal Chemical. The foliar material is also registered for powdery mildew or other disease control on a variety of crops including apples, pears, grapes, cherries and strawberries.

Its active ingredient, triflumizole, can be used in all fungicide-resistance management strategies, especially alternation with strobilurins, a separate class of fungicides. It also has a zero-day preharvest interval on melons and related crops.

Alternating a must

Powdery mildew and other fungi develop resistance when repeatedly exposed to control products having the same mode of action, so alternating or rotating materials is a must.

“In our work on grapes Procure did not show the degree of resistance as other, older DMIs such as Bayleton. However, the key to preventing resistance to this class of materials is to not rotate DMIs with one another, even if a product shows more activity than a rotational product,” Gubler stresses.

Good rotation partners for Procure would be strobilurins such as Flint or Quadris, he adds. Materials with different modes of action can be alternated within a season to good effect, as long as applications of materials having the same mode are not consecutive.

“For instance, you could use a application of Procure, followed by a strobilurin, and then Rally (a DMI). Or you could go with Rally, followed by a strobilurin, and then Procure. In any case, you do not want to go with two or three treatments of Rally and then two or three of Procure.”

Stretch intervals

The next season the grower can repeat the same pattern if it proves satisfactory. Or he can switch the product order, while still alternating modes of action. “If a grower has figured out how to rotate products to get efficacy, then he can stay with it the following year.”

Using the risk assessment index to get an idea as to what to expect, Gubler says PCAs and growers can stretch their fungicide application intervals accordingly to conserve material and application costs.

However, he reminds, even with benefit of the index and/or varieties having some resistance, growers should diligently scout for the white, dusty signs of the disease.

Ideally, Gubler contends, growers should use high-pressure spray equipment with drop nozzles and ceramic tips to circulate a mist throughout the foliage, especially the undersides. Regardless of the spray equipment used, tractor speed should be slow enough to provide complete coverage.

San Joaquin County Farm Advisor Benny Fouche' rates Procure at the top in his ongoing, comparative trials with fungicides for powdery mildew control on spaghetti squash, one of the most sensitive of cucurbits, and he encourages the disease with dense plant spacings with high moisture and high fertilization.

Growers in areas with a history of the disease can apply low rates of Procure as “a base of protection,” even before foliage is fully developed, according to Stephen Colbert, Crompton/Uniroyal Chemical product development manager in Fresno. The label rate is 4 to 8 ounces per acre, for a maximum of 40 ounces per acre per season.