Researchers seek solutions as diseases grow tolerant to fungicide

01/15/2014 05:00:00 AM
Tom Burfield

Odell typically uses Bravo, but adds products like Ranman and Curzate when late blight shows up.

“We’ve got four or five different materials that we use in rotation to control late blight,” he says. “It’s been a fairly successful program for us. We haven’t had any wipeouts.”


Brown rot

Molly Giesbrecht, Texas A&M AgriLife Extension; bugwood.orgIf you see brown rot in a well-maintained peach orchard, chances are the disease has become resistant to many of the commercial fungicides. Brown rot symptoms include rotten or mumified fruit or half rotten fruit with brown lesions. The lesions may have fuzzy brown fungus on top of them.Typically, you won’t find brown rot in peaches in a well-maintained orchard, says Guido Schnabel, professor of plant pathology at Clemson University. But if you find greater than 5 percent to 10 percent, he says, “You can probably guess that there might be resistance going on.”

Brown rot likely is present if fruit is completely rotten and mummified or perhaps half rotten with brown lesions and a “fuzzy brown fungus on top of the lesions,” he says.

Together, Georgia and South Carolina produce more peaches than any other states except California, he says.                 

“We have seen resistance in both states in most of the chemical classes.”

Growers have commonly used the pesticide Orbit but now seem to favor a similar product called Tilt to combat brown rot, he says. Propiconazole, a DMI fungicide and Tilt’s active ingredient, also is available under other brand names.

Growers are seeing resistance to Orbit and Tilt-like products and also to Topsin-M, which has the active ingredient thiophanate methyl—a different chemical class, Schnabel says.

There also is a combination product of two chemical classes—QOI fungicides and SDHI fungicides—called Merivon.

“It’s a new product that just came out last year, Schnabel says. “It’s really good for brown rot control and for strains that are resistant to Topsin M and Orbit.”

If brown rot is resistant to propiconazole, you still can control the resistant strain with a high rate of a similar-type product, such as Indar, he says.

You can’t increase the rate of propiconazole because of residue issues, but other members of the same chemical class can be increased.

Fenbuconazole is the active ingredient in Indar.


Challenging times

Mike DuBose, partner in Cotton Hope Farms in Monetta, S.C., has been having an increasingly difficult time finding a chemical to control rots.

“We are beginning to see resistance to everything that is available now,” he says. “We’re having to rotate different products to combat resistance.”

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