Researchers seek solutions as diseases grow tolerant to fungicide

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

DuBose, whose family has been growing peaches for more than 100 years, says that in the past, new products would come along to replace those to which rot had developed a resistance.

That’s not the case today, he says.

“We’re just rotating all of the chemical classes that we have, so that we can take care of the ones that are resistant to one chemical class,” he says.

The good news is some new products are being developed, he says, and some existing products soon may be registered for use on peaches.


Gummy stem blight

David Langston, University of GeorgiaGummy stem blight, one of the most common diseases of melons, has become resistant to several fungicides. To slow it from happening to the remaining effective products, tankmix or rotate products with different modes of action.Many fungicides are used to combat gummy stem blight in watermelons because there are no resistant cultivars and cultural practices don’t offer a lot of protection, says David Langston, professor and Extension vegetable pathologist for the University of Georgia.

Symptoms of gummy stem blight, the most widespread watermelon disease, are large, irregular-shaped brown lesions on leaves and water- soaked brown lesions on stems.

“Severe infestations could lead to complete defoliation of a crop,” Langston says.

The broad-spectrum protectant Bravo, with the active ingredient chlorothalonil, is the most common fungicide used against gummy stem blight in watermelons, he says. But Bravo does not work as well as some others under high disease pressure.

Bravo also can’t be used safely within 21 days of harvest because it has a history of burning watermelon rinds, Langston says.

Other fungicides are single-site modes of action and present a higher risk of developing resistance, he says.

“Strobilurin fungicides used to work, but we’ve got widespread resistance to those,” Langston says. “Those just don’t work anymore.”

Nor do the benzimidazoles, such as Topsin M and the now-discontinued Benlate, he says.

Many growers use DMI fungicides, such as tebuconazole and some others, because they are off patent.

Inspire Super is a different type of DMI that is premixed with fungicide from a different class and “works really well,” Langston says.

Another group, SDHI fungicides--such as Pristine and Fontelis, used to be very effective, but growers have seen a lot of resistance to them in recent years, he says.

Luna is an SDHI that works well and has no resistance, Langston says.

 “That’s probably the most effective fungicide used against gummy stem blight today,” he says. “We don’t see cross resistance between Luna and the other SDHIs.”

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