Powdery Mildew Control in Oregon Hops: The (Pint) Glass is Half Full

HopsWhen the fungal disease powdery mildew first appeared in hop yards in Oregon in the late 1990s, it was devastating from both a production and integrated pest management standpoint.

“The first thing everybody did was just start spraying everything, whether you had the problem on your farm or not,” remembered grower Fred Geschwill of F&B Farms in Woodburn. “We were spraying quite a bit of sulfur and a couple of wide-spectrum fungicides – probably ineffectively and at the wrong time.”

The good news is that in the 20 years since that initial outbreak, researchers and growers have learned a lot about the disease and how to manage it. Just in the past few years, in part through research funded by the Western IPM Center, fungicide applications in Oregon hops have dropped about 40 percent.

However, as researchers like David Gent of the U.S. Department of Agriculture’s Agriculture Research Service continue to study the problem, they’ve shown that early season cultural practices could essentially eliminate the disease from Oregon hops farms.

Links & Resources
See the video
IPM improvements in hops
Controlling Prionus beetles
Hops IPM Field Guide
Oregon Hops Commission

“There’s a bottleneck in the lifecycle of this pathogen where we can really affect it,” Gent explained. “The more growers adopt these cultural practices, the more initial infections we can eliminate and the more we can suppress disease industry-wide.”

Put more simply, there’s an areawide solution for powdery mildew. But everyone has to be on board for it to work, and the industry isn’t quite there yet.

For powdery mildew in hops, the pint glass is only half full.

A Problem that Spreads

A mildew-infected flag shootThe type of powdery mildew fungus that’s present in Oregon only survives and overwinters in plant tissue. In hops, which are a perennial crop, the fungus first reappears in the spring as “flag shoots,” early sprouting shoots that are infested with the fungus. They emit spores which float on the wind and can infest clean plants more than a dozen miles away.

“We know that the fungus overwinters in just a handful of fields,” Gent said. “It starts out at a very low level, in about five percent of the fields. Then it spreads to affect one-third to two-thirds of the hops fields.”

Gent and his team inspect every hop field in Oregon, most of which are concentrated in the Willamette Valley north of Salem. They’ve mapped the disease throughout the growing season and have seen four or five initial early spring discoveries spread across field after field as the season progresses, blown by the prevailing winds.

“The maps really tell the whole story,” Gent said. “What happens in a very small number of fields scales up and has huge implications for how the disease develops at the landscape level.”

But here’s what growers can do – cut off those first flag shoots and eliminate the initial source of powdery mildew before it even appears. And there are age-old cultural techniques in hops, known as crowning and scratching, that do just that.

In crowning, the top inch or two of the plant’s root crown is cut off by a blade spinning just below soil level. Scratching uses a disc to scratch the top surface of the soil and remove buds from the crown. The practices used to be standard in the industry, both for disease control and because they promote uniform emergence of hop shoots, which maximize yields.

But as fields and farms grew larger, and growers installed drip irrigation, crowning was replaced with the use of herbicides to kill those early shoots, a practice growers call burning.

Hops being crowned“The old timers had all this stuff figured out,” said grower Kevin Crosby of KW Crosby, Inc. in Woodburn. “I mean, my dad would roll over in his grave if I was out there just burning my hops and not crowning. Before we had these systemic chemicals, growers had to do a really good job of keeping their fields clean. Today, I think, growers in general run a lot more acreage and rely on spraying too much.”

Reasons Not To

There can be reasons not to crown. It can damage newly planted fields, and some growers believe some varieties don’t tolerate crowning well. There’s an expense to crowning every plant in every row in every field, and some larger farms may simply not have enough time to crown every plant in the spring. Drip irrigation lines have to be rolled and hung out of the way, then returned to the field.

And doing all that doesn’t guarantee a thing, because if your upwind neighbor didn’t do it, you may still end up with a powdery mildew outbreak. That makes Gent’s ongoing census of hop disease and those maps all the more critical.

“It’s important to be able to look beyond our farm because those nasty spores travel,” said Gayle Goschie of Goschie Farms in Silverton. “We do what we feel is the exact right thing to do to treat for the disease every year, but if our neighbors are not following suit we need to know. We could think we’re fully in a safe zone, but neighboring spores could put us in danger.”

Another thing Gent’s research shows is that powdery mildew is most likely to appear in fields that had bad outbreaks last year.

“So we’re recommending that if you can’t crown your whole farm, be strategic about what you do crown,” he said. “Target the fields that had powdery mildew last year, and the fields adjacent to those.”

Now the Good News

Growers have started to adopt the practice, and powdery mildew has been declining. From 2014 to 2016, the average number of fungicide applications growers were making dropped from 5.5 a season to 3.5 – a 40% reduction – and powdery mildew infections declined from more than 60 percent of fields to half that number.

Crosby spent a few seasons selectively crowning based on the susceptibility of the variety to the disease and the sensitivity of the variety to the procedure, but then had a field blow up with infection.

“I just said, ‘Enough,’” he said. “I now have a policy written in my policy book that says every hop on this farm gets crowned, is scratched and we just do it. That’s just our 10 commandments.”

Crosby doesn’t see crowning in direct cost-benefit terms, calculating that by crowning now he’ll save so much later in fungicide applications. While his farm is Salmon Safe certified and he feels reducing sprays is good for the farm and its hops, he sees crowning as simple insurance.

“It significantly reduces our risk factors of a crop failure,” he explained. “It’s risk management. I look at it as the same thing as fire insurance on the hop kiln or anything else.”

Areawide Adoption

While there isn’t yet universal adoption of crowning in Oregon hops, there is the possibility it could happen.

The industry is small enough – under two-dozen hop-growing families – and there are examples they can look to of successful areawide programs that required every area grower to join together. (This story, about Lygus bug management in California safflower, is a good example.)

It takes a shift in thinking about competition and cooperation, and acceptance of the idea that less disease is good, but no disease is better.

And attitudes are changing, albeit slowly.

“I think we’re learning as a community to start trying to work with each other and remove the stigma of the disease being looked at as a grower’s failure,” Geschwill said. “In the beginning, there was very little buy-in into this concept and people were really concerned about their perception in the community.”

Now, as growers recognize that what happens on one farm can affect them all, trying to keeping an outbreak quiet isn’t as acceptable as it once was.

“I think there’s definitely a stigma to not telling,” Geschwill said.

And while there may be more grower-to-grower discussion of disease, it’s Gent’s efforts keeping all the growers informed at the moment.

“What would happen if Dave and his team were no longer doing that part?” wondered Goschie. “That’s when the growers will need to sort out how we’re going to do it as a region, and I don’t know if there’s one of us who has really seriously thought about that yet.”

Crosby thinks there’s movement toward an areawide approach but it’s not happening fast enough.

“I think if everyone carried out a program of crowning and scratching and burning, I don’t think we’d have mildew down here,” he said. “That’s my opinion. It sounds pretty simplistic, but I don’t think it’s that hard to control.”