Are Oregon’s blueberry crops at risk from powdery mildew?

A new strain of powdery mildew fungus that attacks blueberry plants has plant pathologists concerned about its potential to spread to the Pacific Northwest, threatening crops and backyard plants.

One strain of the fungus, Erysiphe vaccinii, was initially contained to its native region in the Northeast, but is circulating in Morocco, Peru, and Portugal. A different strain has spread to China, Mexico, and California, raising alarms about a potentially serious threat to global blueberry production. The disease causes leaves to be covered with a white powdery substance that takes nutrients from the host plant and reduces photosynthesis, thereby reducing blueberry yields. 

“It could be really serious,” said Wei Qiang Yang, a professor of horticulture and blueberry expert with Oregon State University who is monitoring the situation. “It is spreading to different parts of the world. It seems it’s a new type or species of this strain of mildew.”

The fungus appears to be a mutation of an existing powdery mildew pathogen with concerning new characteristics. While powdery mildew diseases are prevalent in grapes, blueberries have typically been more resistant due to their thicker, waxier leaf structure, which the mildew can’t penetrate as easily. Different species of the fungus are affecting wheat, hops, grapes, and strawberries.

“The new strain, from what I’m seeing, prefers strong day and night temperature differences and about 50% humidity,” said Yang. “If it has those favorable conditions where it can survive, it could spread.

“We have those conditions.”

Michael Bradshaw, assistant professor of plant pathology at North Carolina State University, is primary author of a paper describing the disease published in January 2025 by the journal New Phytologist. Bradshaw explained the fungus is not new, but is now changing and spreading. 

“It’s native to the Northeast, so it’s been common there, but within the past 12 years it’s now spreading throughout the world,” he said. 

“This pathogen poses a considerable risk to the blueberry industry by reducing crop yields and increasing reliance on fungicides,” Bradshaw wrote in the paper.  

What makes this strain particularly worrisome is it has developed both sexual and asexual reproduction capabilities in the United States, while in China it currently only reproduces asexually.

Oregon a top producer of blueberries

There are two varieties of blueberries — the wild lowbush and the cultivated highbush — and both are native to North America, according to the U.S. Highbush Blueberry Council. 

The first highbush blueberry bush was successfully cultivated for commercial production in the early 1900s, and today, nearly 1 billion pounds of blueberries are grown in North America each year. 

Oregon is one of 10 states considered to be a top blueberry producer. In 2023, Oregon growers cultivated 13,700 acres of blueberries, according to the 2024 Oregon Annual Statistics Bulletin, compiled by the United States Department of Agriculture.

In Oregon and Washington, as in North Carolina, commercial blueberries are primarily grown outdoors rather than in tunnels or greenhouses. 

“In China, they grow a lot of their blueberries in tunnels, hoop houses, and in greenhouses, which increase the damage of the disease,” Bradshaw said. “That’s why I hypothesize that we see more damage in that area. In Oregon and Washington, where it’s mostly grown outdoors, my hypothesis is that if the fungus gets to that area it won’t cause the same amount of damage as it would in some other places in the world.”

Bradshaw said recent research is a good first step and future investigation will involve breeding efforts to try and introduce resistance to the pathogen.

“This is new, so we are learning as we go,” Bradshaw said. “And we don’t know enough to know yet whether the fungus reaching its sexual stage means it will be more virulent or not. There just hasn’t been enough research to answer that question.” 

Northwest risk assessment

There have been no reported cases in the Pacific Northwest. However, Yang and Bradshaw suggested vigilance is necessary. 

“The good thing is that we are not getting blueberry plants here from the eastern part of the country, especially North Carolina,” Yang said, noting that this could help prevent plant-to-plant transmission. 

Human vectors remain a concern, however. 

“We have people going into different fields and then coming back to our fields,” Yang said, suggesting spores could travel on shoes or clothing of people who visit affected areas. 

The fungus manifests as a white powdery substance on blueberry leaves, making it relatively easy for growers to identify.

If you spot blueberry powdery mildew

Yang and Bradshaw encourage backyard growers and agricultural professionals to stay informed about the situation and maintain communication channels with affected regions to monitor the pathogen’s spread and development. 

Northwest blueberry growers who spot symptoms are asked to immediately report potential infections to OSU specialists who can collect samples for confirmation, or to Bradshaw for tracking purposes. 

If you spot symptoms, contact Wei Qiang Yang at wei.yang@oregonstate.edu or Oregon State University’s Extension Service at https://extension.oregonstate.edu, or Michael Bradshaw at North Carolina State University at mjbradsh@ncsu.edu.