New fungicide products are strengthening resistance management programs, but growers still face a tough fight.
Even growers with a good range of fungicide options must take resistance management concerns seriously.
"If we don't use them right, we will lose them down the road," says Doug Gubler, a University of California Extension plant pathologist in Davis.
Gubler works with powdery mildew in grapes, where the development of fungicide-resistant isolates has complicated the use of many controls.
"You can still use them effectively, but you have to know the disease pressure," he says.
War on apple scab continues
Meanwhile Midwest and East Coast apple growers are just holding their own against apple scab.
"I can't see it getting any better," says Janna Beckerman, associate professor of botany and plant pathology at Purdue University, West Lafayette, Ind. "At best we're maintaining some sort of dynamic equilibrium."
For the near future, she sees no new chemistries to relieve the strain on the DMI and strobilurin fungicides.
Strobilurin-resistant apple scab is a relatively new, and widespread, problem, but recent tests found about 12 percent of the pathogen's isolates resistant to the four main fungicide classes.
New fungicides registered for use against apple scab rely on the same modes of action as those existing controls, hindering the basic resistance management practice of rotation through different classes of chemicals during the growing season.
"We haven't had any new stand-alone (products) since the strobilurins came out in the '90s," Beckerman says.
"For the most part we don't have active systemic fungicides," says George Sundin, professor of plant pathology at Michigan State University in East Lansing.
What's left for the state's apple growers are broad-spectrum protectants that must be applied before outbreaks and that require more frequent sprays. Use limits through the season also come into play, he says.
Some of the new products, including Inspire Super (a mix of difenoconazole and cyprodinil) and Topguard (flutriafol), are DMIs, and may not persist long before resistance problems appear, he says.
Storm waters ahead?
Others in the registration pipeline include products in the SDHI fungicide class that Sundin says don't provide optimal scab control.
"We're entering a potentially problematic period for the Midwest and Northeast," he says.
Still, apple growers can take steps to reduce risks.
"Make sure you actually have resistance," Beckerman says.
She tests samples for resistant pathogens to help guide growers' spray programs. Spray early and often enough to provide adequate control.
A recent study indicates that an early-season copper spray may reduce fungicide-resistant isolates.
At the same time, growers whose blocks show high inoculum counts should shift from systemic products to protective sprays such as Captan until disease pressure drops, she says.
Rescue programs under those conditions will only create bigger long-term problems, she says. Greater numbers of resistant survivors are likely to overwinter and reproduce.
"It means one tough year but it will bring (the situation) back to a more manageable level," she says. After harvest, mow between rows and break down leaves with urea applications, Sundin says.
That speeds elimination of the pathogen's food source and helps reduce the next season's sporulation.
Beckerman also suggests planting scab-resistant cultivars when changing over orchard blocks.
For now, Honeycrisp is still fairly resistant, while McIntosh is so susceptible it serves as the canary in the coal mine, the first trees in an orchard to show signs before apple scab radiates outward.
If the worst happens and you're hit hard by a disease epidemic, think twice before turning to sprays with resistance concerns.
"It's a numbers game," Wilcox says. "Ideally you don't use at-risk materials in a rescue mode."