New Fungus Causes Hop Disease Across Lower Peninsula

A new species of fungus is threatening Michigan’s hop industry, according to researchers at Michigan State University in East Lansing. While craft brewers in the state are not facing a hop shortage, they prefer to use locally grown hops.
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hop with halo blight
MSU researchers are studying a new fungus that causes what they call halo blight, shown on the pictured hop. // Photo courtesy of Michigan State University

A new species of fungus is threatening Michigan’s hop industry, according to researchers at Michigan State University in East Lansing. While craft brewers in the state are not facing a hop shortage, they prefer to use locally grown hops.

Working with hop growers in the Lower Peninsula, researchers from Michigan State University, MSU Extension, and MSU Plant and Pest Diagnostics have identified the new pathogen, which infects hop leaves and cones — the flowers used in brewing.

According to the MSU team’s Aug. 25 publication in the journal Plant Disease, growers have estimated losing between 20 percent and 50 percent of their yield to the fungus.

The MSU researchers are working on strategies to manage what they’re calling halo blight, the disease caused by the fungus. Michigan is the nation’s fifth largest producer of hops, but the Pacific Northwest grows most.

“This isn’t going to threaten our supply of hops, but it hurts our ability to supply locally grown Michigan hops,” says Timothy Miles, an extension specialist and an assistant professor in the Department of Plant, Soil, and Microbial Sciences. “In Michigan, we are proud of our craft beer industry, and I think we’d like to get all of our hops in Michigan if we could.”

The state of Washington is the largest producer of hops in America, with 40,880 acres under cultivation in 2019 — more than 69 percent of the total in the U.S., according to the Hop Growers of America. The other top states include: Idaho second at 8,358 acres; Oregon third at 7,306 acres; Wisconsin fourth with 1,297 acres; and Michigan fifth with 720 acres.

The work to identify the new disease began in the summer of 2018. Michigan hop growers noticed that some of their plants had brown splotches on their leaves.

The growers sent a sample to MSU plant pathologist Jan Byrne at MSU Plant and Pest Diagnostics, who examined the lesions and determined the disease was caused by a fungus.

“I knew it was something new, something I hadn’t seen on hops before,” she says. After receiving another sample from another hopyard, “We knew this was more than a one-time thing,” she says.

Doug Higgins, Ph.D. student, extracted DNA from the fungus and compared its genes to known hop pathogens. The fungus has several close, but no exact, matches, confirming it is a new species. The scientific name of the fungus is currently Diaporthe sp. 1-MI.

The researchers then collected hundreds of leaf and cone samples from more than a dozen commercial hopyards across the Lower Peninsula as well as from MSU’s Plant Pathology Farm to find out how widespread the fungus is. They found the fungus accounted for nearly 40 percent of all the fungal samples the team collected.

They also suspect the hop blight is showing up throughout the Great Lakes region and other locations with wetter, more humid climates than the growing regions out west.

“Now there are all sorts of questions that are relevant to management,” Higgins says. “How does it get around? Where does it come from? Now we can be more targeted in how we answer those using what we’ve learned.”

The team is also studying whether fungicides already approved for use in hopyards are effective against the fungus and expects to have information to share with growers over the next few years.

In 2019, the National Brewers Association reported there are 400 craft breweries and brewpubs in Michigan, ranking it sixth in the nation.

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