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."