A new strain of powdery mildew has overcome a resistance gene in hops, allowing the fungal pathogen to spread in high levels across Oregon and Washington.
Farmers have discovered the disease in more than 60 percent of surveyed Oregon hopyards planted with varieties that previously conferred resistance to the fungus, and nearly 100 percent of Washington hopyards, experts say.
“It’s significantly more than we’ve seen historically,” said David Gent, research plant pathologist with USDA’s Agricultural Research Service.
Not all of those hop yards were necessarily damaged by the powdery mildew, however.
Growers are able to control the disease with fungicides, but the additional sprays have increased expenses by about $80-$100 per acre, raising the cost of production by 3 to 4 percent, said Fred Geschwill, a farmer from Woodburn, Ore.
“It’s a serious issue with cost of production,” he said.
The market for hops is currently strong enough to bear the added cost, but the problem would be aggravated if there was another industry downturn, Geschwill said.
“The scariest part is it changed overnight,” he said, referring to the strain’s virulence and resistance. “It’s like the foot’s on the gas.”
Traditionally, downy mildew was the biggest threat to Oregon hops, but growers now face an equal risk from the new strain of powdery mildew, Geschwill said.
Killing off the hop’s crown buds early in spring can reduce the amount of powdery mildew that’s harbored in fields over winter, but the efficacy of the practice is reduced if the resistance gene no longer works, he said.
“If there’s no gene helping you out, you can’t expect cultural practices to help you as much,” he said.
The gene that conferred resistance to hops, known as R6, managed to help stave off the disease for 14 years, which is pretty long for powdery mildew, said Gent.
The fungus likely overcame the resistance through selection pressure, which allows pathogens with a genetic advantage to spread, he said.
John Henning, research plant geneticist with USDA ARS, said he and Gent have been inoculating test varieties with powdery mildew to identify tolerant varieties.
It’s too early to speculate about the success of these efforts, as the new strain of powdery mildew has only appeared this year, he said.
Rather than relying on a single gene to confer resistance, the goal would be to develop “quantitative resistance” from multiple genes, Henning said.
Such resistance may be achieved by selecting hops that appear to withstand the disease or are less affected by it, he said.
The process may be aided by identifying “molecular markers” associated with resistance, Henning said.