Janna Beckerman, an associate professor of botany and plant pathology art Purdue University, said extensive, long-term use of four popular fungicides has led to apple scab resistances in apples in Indiana and Michigan.
Apple scab, caused by the fungus Venturia inaequalis, is highly destructive, causing brown lesions on leaves and fruit of apple trees, a Purdue release said Tuesday.
"The fungicides that are regularly used to control scab have started to fail," Beckerman said.
"But the most disturbing thing we found is that many of the samples we tested were resistant to all four fungicides. It's kind of like multidrug resistance in antibiotics. This is full-blown resistance."
"It can cause orchard failures," Beckerman said. "An orchard grower that has this could lose blocks of an orchard, or depending on the amount of diversity in the orchard, they could lose the entire crop."
The only option, she said, is to use older fungicides that are tightly regulated, require more frequent application and are more expensive.
"It's going to change how growers manage their orchards," Beckerman said. "The more susceptible apple cultivars, like McIntosh, will become more chemically intensive to manage.
"Growers have few options as it is, and this will limit their options even further."