Rotate chemicals with cultural controls to thwart resistant brown rot in stonefruit

By Marni Katz

Reduced brown rot sensitivity to a popular class of fungicides in Georgia peaches should be a wake-up call to stonefruit growers across the country, say university plant pathologists from the Southeast. If the fungicides, known as demethylation inhibitors or DMIs, aren’t used judiciously, their future effectiveness against the fungal disease may be in jeopardy.

“We do have confirmed reduced sensitivity and have verified this for several years through 2005,” says Guido Schnabel, an assistant plant pathology professor at Clemson University in Clemson, S.C. “We can’t call it resistance per se, because you do not find 100 percent failure when you spray the fungicide. There is some effect, but the effect is just not very good.”

In response, Georgia Extension specialists have revised recommendations to include new fungicides that they hope will address resistance and slow brown rot pressures.

Although growers elsewhere may not face the same problems, experts say they still can benefit by taking similar steps to manage resistance and retain the effectiveness of newer fungicides that act on a single site within the organism.

For more than a decade, DMIs have been a mainstay against brown rot, the leading stonefruit disease problem around the world.

The fungus Monilinia fructicola is responsible for early-season blossom blight that later develops into fruit rot.

As Monilinia developed resistance to older fungicide classes, including benzimidazoles, during the 1980s, DMIs replaced those fungicides.

Because the fungicides act on one control site, the organism can easily overcome the chemical. Several DMI fungicides are registered nationally for blossom blight or fruit rot, including Orbit, Indar, Nova and Elite. Resistance to one fungicide leads to cross resistance to all fungicides within the same class.

Products lose effectiveness

Beginning in 2003, growers in middle Georgia, where most of the state’s peaches are grown, started to notice that their standard program of three applications of propiconazole or other DMIs wasn’t controlling brown rot.

Spurred by grower reports, Schnabel and University of Georgia Extension plant pathologist Phil Brannen began testing Monilinia isolates from Georgia’s orchards for reduced sensitivity. A more extensive survey in 2004 found that many of the fungus isolates showed reduced sensitivity to propiconazole.

Brannen says one block in 2005 showed signs of full-blown resistance, indicating the problem was getting serious.

“Under heavy disease pressure, (DMIs) are not doing well in that region, although under low disease pressure they probably would stand up all right,” says Brannen, who’s based in Athens, Ga.

Schnabel says brown rot testing in other Southern peach production regions has determined that reduced DMI sensitivity is restricted to middle Georgia.

“Why that is is a question asked a number of times, but we don’t really have a good theory,” Schnabel says. “It’s not that growers sprayed more. It may be that the weather conditions are more suitable in Georgia for the disease development, so the fungus has better conditions to adjust to the stress of fungicides.”

Rotate, rotate, rotate

Since the 2004 survey, Georgia Extension specialists have revamped their brown rot fungicide recommendations to emphasize strobilurins, which are respiratory inhibitors.

Like DMIs, strobilurins are site-specific and highly prone to resistance and should be managed accordingly, Brannen says.

“In 2005, we had the second-wettest year on record in Georgia, and we should have had tremendous brown rot pressure,” he says. “Growers who switched to this new program with strobilurins did not experience substantial brown rot pressure. It was a near miracle.”

Schnabel and Brannen recommend growers alternate fungicide classes. They say DMIs and strobilurins should be preserved for preharvest applications and should not be used during bloom for blossom blight control or during cover sprays. Growers instead should use products such as Vangard and Bravo for blossom blight.

“If they spray twice during the preharvest season, they should use the respiratory fungicides for the first spray and follow up with the DMIs for the second,” Brannen says. “The important thing is to rotate, and it is very important to start with the strobilurins for the preharvest sequence of sprays.”

When benzimidazole resistance is not present in an orchard, Topsin M can be an effective rotational tool, Schnabel says. It should be applied only once per season in a tankmix with a protectant, multi-site fungicide such as Captan.