Tomatoes targeted

06/01/2008 02:00:00 AM

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.

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