Tomatoes targeted

06/01/2008 02:00:00 AM

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.

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