The research indicates that the fungus, Erysiphe vaccinii, has expanded from its initial area in the eastern United States to various continents over the past 12 years.

“We are currently observing this gradual worldwide spread,” stated Michael Bradshaw, an assistant professor of plant pathology at NC State and the lead author of the research paper.

As the name suggests, powdery mildew disease manifests as a white, powdery coating on affected plants, which siphons nutrients and hampers photosynthesis without killing the host. Various plant species, including wheat, hops, grapes, and strawberries, have suffered from the effects of powdery mildew.

“There are even other related powdery mildews that impact plants like wild berries or eucalyptus. However, these are genetically distinct from those affecting blueberries globally,” Bradshaw explained.

In their study, Bradshaw and his team analyzed historical and contemporary plant leaves afflicted by powdery mildew. Their collection encompassed 173 samples sourced from North America, Europe, Africa, and Asia; one of the North American samples was noted to be over 150 years old, while the international samples were collected in the last five years. The first identification of powdery mildew outside of North America occurred on a farm in Portugal in 2012, according to one of Bradshaw’s co-authors who previously worked for a major berry producer.

To understand the history and propagation of powdery mildew disease, the researchers conducted genetic analysis on the fungal samples. Notably, none of the historical specimens shared genetic similarities with the strains currently expanding globally.

The findings revealed that the disease started in the eastern U.S. and was disseminated worldwide through two separate introductions. One strain of E. vaccinii reached China, Mexico, and California, while another strain spread to Morocco, Peru, and Portugal. Bradshaw suspects that human activity, particularly the movement of nursery plants, facilitated this spread.

“Controlling this organism is challenging,” Bradshaw remarked. “When you ship plant materials internationally, you’re likely dispersing this fungus along with it.”

Interestingly, the research found that the E. vaccinii fungus present in blueberries from other countries seems to reproduce only asexually, differing from the way it reproduces in the United States where both sexual and asexual reproduction occurs.

The study also collaborated with a major company and farmers to estimate the global financial impact of powdery mildew on blueberries, particularly the costs associated with fungicide application to manage or reduce the disease. The analysis suggests that this economic burden ranges between $47 million to $530 million annually for the worldwide blueberry sector.

Lastly, the research highlights potential warning signs for key blueberry cultivation regions, such as the U.S. Pacific Northwest. Although conditions there are conducive to the spread of powdery mildew, the disease has not yet established itself in that area.

“Agricultural conditions can also influence the spread of the disease,” Bradshaw noted. “For instance, areas that cultivate blueberries in tunnels or enclosed environments tend to experience worse disease outcomes compared to those that grow blueberries outdoors in locations like North Carolina.”

Bradshaw emphasized that the researchers developed a valuable tool to help identify E. vaccinii strains, assisting both farmers and scientific researchers.

“Identifying the fungus responsible for powdery mildew in blueberries can be complicated, so we submitted our data to a public database created at NC State by co-author Ignazio Carbone. This platform enables growers to input their data and determine the specific strain present in their fields,” Bradshaw explained. “This is crucial as understanding the genetics of the strain can inform farmers about its potential resistance to fungicides and the nature of the disease’s spread, including the virulence of different strains.”

The full research paper is published in New Phytologist. Funding for the study was provided by the National Science Foundation with awards numbered 2315953, 2200038, 2031955, and 2308472. Additionally, support came from the Novo Nordisk Foundation with grant numbers NNF19SA0059360 and NNF19SA0035476.