Improved brown rot test helps ID fungicide resistance in stone fruit

05/01/2011 09:52:00 AM
By Renee Stern, Contributing Editor

A quick test for fungicide-resistant brown rot disease in peaches also may aid strawberry growers struggling with similar fungal diseases.

Guido Schnabel, associate professor of fruit pathology at South Carolina’s Clemson University, says the same technique used in the brown rot test should translate into a system for anthracnose and botrytis fruit rots in strawberries.

An $850,000 U.S. Agriculture Department grant will fund his strawberry work, which includes developing a resistance monitoring tool and implementing a spray application decision support system.

The grant is part of a larger multi-university project to improve management of both diseases.

For brown rot, fungal samples collected in the field with special test kits show within 72 hours any sensitivities to specific fungicide classes.

Used for the past three years in Southeastern peach orchards, the test should work equally well for any pathogen that sporulates, grows on artificial medium and develops early in the season, Schnabel says.

Among them would be gray mold of grapes and brown rot infections in cherries and other stone fruit, he says.

The technique works best with fungi whose spores are easily collected and easily grown in a Petri dish.

Unfortunately, that rules out a similar test for downy and powdery mildews, says Phil Brannen, associate professor of plant pathology at the University of Georgia in Athens.

A more user-friendly test

Schnabel and Brannen worked together to develop and improve the brown rot test, making it more user-friendly.

A new online application—viewable at http://bit.ly/gx6qhN—delivers test results with spray recommendations tailored to individual resistance situations.

Their work has helped growers fine-tune their spray programs with a more accurate sense of risk versus cost, Brannen says.

That’s given growers more confidence and fewer worries about brown rot, says Lawton Pearson, a partner in Pearson Farm in Fort Valley, Ga.

“The biggest pathogen and fight for us is brown rot,” Pearson says.

Some years are worse than others, with more favorable conditions to the disease. Six years ago a severe outbreak of fungicide-resistant brown rot—developed through continued reliance on sprays from a single chemistry class—hit home with the need to rotate modes of action, he says.

“In one block, half the peaches had to be put on the ground,” he says.

The tests not only ended the uncertainty about which blocks held resistant brown rot, but also helped target the orchard’s fungicide use.

“We may not be spending less money, but we get 100 percent control,” Pearson says. “We have enough bullets [in the spray program] if we know when and how to use them.”

Resistance-management strategies

Standard protocols to control brown rot are too general, given microclimates that create different disease conditions across even a small region, Schnabel says.

And individual spray patterns in previous years can foster different resistant fungi populations from farm to farm.

“Growers want location-specific information,” he says.

Ideally, widespread use of the resistance test will reduce the need for fungicide sprays through more targeted use, he says.

“It’s a tool to develop a long-term resistance strategy,” says Greg Henderson, a Clemson Extension agent for commercial fruit in Edgefield, S.C. With it, growers can correct missteps in fungicide programs before they become critical mistakes.

Resistance will persist in blocks once it gets a toehold, requiring a complete management change for affected areas, he says.

Results over the past two years hint that resistance to one class of brown rot fungicides—demethylation inhibitors such as propiconazole and metconazole—may be diminishing, Brannen says. That’s a difficult conclusion to validate, however.

At the same time, a trend toward strobilurin resistance is surfacing in southern Georgia orchards, he says. The test kits allow close monitoring to stave off serious problems there.

That monitoring capability also allows growers to check their orchards throughout the growing season.

Should any resistance issues pop up, they can amend their spray programs immediately, he says.

How many samples?

One question still to be resolved is how many samples are needed to provide an accurate snapshot of an orchard’s brown rot population, Brannen says. Current recommendations call for sampling 10 peaches.

“There is potential for false negatives,” where sampling might miss hot spots with resistant fungi, he says. Take samples in multiple spots rather than from a single tree.

The tests also may report false positives, most likely due to contamination when collecting samples.

“The results are only as good as the samples you take,” Henderson says. “It takes a lot to mess up, but you can mess it up.” Although careful handling to keep the swab and sample tube clean is critical, “You can train anyone to use this kit in an hour,” he says.

The researchers plan another year of screening and improving the test before releasing a final version to Extension agents and growers for their use, Brannen says.

“I’d like to see [the test] in a form that I could do it myself,” Pearson says. His experiences so far have sold him on its value.

“There’s a lot to be said for sleeping at night when you feel confident your peaches are protected,” he says.



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