Powdery mildew woes heighten need for resistance management plan

By Vicky Boyd


A University of California plant pathologist says he believes two cases of tomato powdery mildew are resistant to two widely used fungicides. Whether those are isolated instances or signs of a more widespread problem in California’s tomato production areas remain unknown, says Michael Coffey, a plant pathology professor at UC Riverside.

“We really don’t know what we have, but we definitely have resistance in these populations from two distinct areas in the main production area,” he says.

Another unknown is whether the mild climatic conditions that were conducive to tomato powdery mildew in 2007 will repeat this season.

Regardless, Coffey and tomato industry officials say the discovery should prompt growers to re-examine and strengthen their fungicide resistance-management programs.

“Whether the tolerance is widespread or whether it’s two isolated cases, growers should still be taking the same resistance-management approach,” says Jim Mueller, a field scientist with Dow AgroSciences based in Brentwood, Calif.

That approach should include scouting for disease, rotating chemistries, tankmixing other fungicides with sulfur, using fungicide labeled rates and timings, and obtaining good application coverage.

Testing for fungicide resistance

The question of fungicide resistance was raised after several tomato growers, particularly those with late-season fields, reported problems controlling powdery mildew in 2007.

Coffey received infected plant samples from two different locations and conducted greenhouse tests to determine whether the organisms were resistant to two widely used fungicides--Rally and Quadris.

In earlier resistance work with other crops, Coffey isolated spores from the powdery mildew and tested them individually for resistance.

Because of the complexity of those tests, Coffey says he opted for a quicker and simpler greenhouse trial.

In that test, he hung the infected plants over potted tomato plants that were treated with Rally or Quadris or left untreated.

He then waited 18 days before rating the plants for powdery mildew infection. Both the treated and untreated plants exhibited powdery mildew symptoms.

Coffey says he ruled out application errors because the untreated plants also were infected with Oidium lycopersicom, a powdery mildew species that most commonly infects greenhouse-grown tomatoes. But the treated plants were not infected.

Had there been an application error, all three of the treatments would have shown signs of the greenhouse powdery mildew, he says.

Because he did not isolate the mildew spores, Coffey says he doesn’t know whether individual organisms are resistant to both fungicides. Individual organisms within the same population could be resistant to one product whereas others in the same population could be resistant to the other fungicide.

“We really don’t know what we’ve got out there—we’re going in blind,” Coffey says.

Stay ahead of mildew

Brenna Aegerter, a UC Cooperative Extension farm adviser in San Joaquin County, says she isn’t surprised by Coffey’s findings, considering tomato growers make repeated fungicide applications and only have a handful of registered products from which to choose.

“We’ve had [Rally] registered for tomatoes since 2000-01, and we’ve relied on it quite a bit in the intervening years,” she says. “Until the strobis came along, we didn’t have too many alternatives. It wouldn’t be shocking with [the DMI] class of fungicides to see a gradual drop in efficacy.”

Strobilurin fungicides only have been registered for tomatoes for the past few years.  

Aegerter blames part of the mildew-control problem on last year’s moderate temperatures, which were nearly ideal for tomato production as well as powdery mildew. Near the end of the season, powdery mildew pressure was abnormally high in many areas.

Unlike some mildews, the species that infects field-grown tomatoes—Leveillula taurica—can remain latent in plant tissue for up to two weeks, causing only mild chlorotic spotting mostly on the underside leaf surfaces. The symptoms are not the stereotypical white or gray fuzzy masses seen with other mildew species in other crops.

Unless you’re familiar with the symptoms and scouting very carefully, you can easily overlook the beginning stages of tomato powdery mildew, she says.

By the time the symptoms are readily visible, the disease is widespread and even the best fungicides have trouble controlling it, Aegerter says.

“They’re very effective if used early on,” she says. “Once the disease is established, you really can’t expect those materials to do a whole lot—they’re more preventative.”

Many growers with late-season fields that had dense plant canopies also relied upon aerial fungicide applications, which don’t provide nearly the coverage that ground applications do, Aegerter says. With powdery mildew, fungicide coverage—especially leaf undersides—is critical.


Take these steps to reduce fungicide resistance risks

Brenna Aegerter, a University of California Cooperative Extension farm adviser in San Joaquin County, and Jim Mueller, a field scientist for Dow AgroSciences based in Brentwood, Calif., provide these tips to reduce the risk of powdery mildew developing resistance to registered fungicides.

Treat early. Applications should begin at the very first sign of disease or even before you see symptoms, if conditions are favorable.

“Powdery mildew has such an explosive disease potential that you have to spray preventatively,” Mueller says.

During the early season, he recommends sulfur at short application intervals of seven to 10 days.

“Begin chemical fungicides, such as Rally, at the first signs of disease or when the forecast model indicates conditions are favorable for disease development,” Mueller says.

He recommends continuing applications on intervals of 14 days or less.

Coverage is critical. Obtain the best coverage possible, and this typically means spraying by ground.

During 2007, many of the performance issues were related to late-season aerial applications to plants with heavy crop canopies and high inoculum levels, Mueller says.

Tankmixes and rotations: Unfortunately, tomato growers only have a few different classes of chemicals that control powdery mildew. To prolong those that work, rotate among different Fungicide Resistance Action Committee groups.

DMIs, such as Rally, belong to Group 3 whereas strobilurins, such as Quadris, Flint and Cabrio, belong to Group 11.

In particular, Group 11 fungicide applications should not be made consecutively, and their use within the season should be minimized.

Tankmixing with a multi-site material—Group M—material is another way to reduce the risk of resistance to Group 3 and 11 fungicides. Dow AgroScience recommends tankmixing all Rally applications with sulfur, a Group M. In addition, Dow recommends tankmixing Rally with sulfur unless environmental conditions may cause crop injury.

Late-season treatments: Once the disease is established in a field, Group 3 and 11 products are less effective. Their continued use under these situations will increase the risk of resistance developing. Once the disease is established, use contact materials, such as potassium bicarbonate (Kalligreen, Armicarb and Milstop, among others) or sulfur (Microthiol and Thiolux). Maximize control with contact fungicides by obtaining good coverage.

Watch your pH: Dow does not recommend tankmixing Rally with foliar nutrients, since it may reduce Rally’s performance, Mueller says. If you need to tankmix with foliar nutrients, Dow recommends first adding those products to the tank, buffering the spray mix pH to between 6.5 to 7.5, then adding Rally.

Although potassium bicarbonate products can help control mildew, they also can increase the spray mix pH. Because of that, Dow does not recommend tankmixing them with its fungicide.

Cultural control: Mueller recommends plowing under tomato fields as soon after harvest as possible to reduce the amount of inoculum.

“Last year we saw harvested fields which had severe mildew. These may have served as inoculum sources for adjacent fields,” Mueller says.

Should you have a fungicide performance problem, contact your local Cooperative Extension office or the manufacturer’s representative.

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