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.