Feared fungal disease has spread to blueberry fields worldwide

It starts as a faint, dust-like layer on the leaves – easy to miss on a busy day in the field, with your eyes on the next row and the next harvest. For blueberry growers, this is often one of the first discreet signs of a disease that drives up costs, cuts yields and pushes them into repeated fungicide spraying to keep it under control.

A new study now shows that a powdery mildew fungus that long seemed to be limited to eastern North America has turned up in blueberry fields well beyond its original range. The researchers draw a new picture of how a plant disease can travel with a global crop – one of the main ways new plant diseases emerge – and why it pays to spot it while it is still only a faint veil.

The work was led by plant pathologist Michael Bradshaw at North Carolina State University, who is also affiliated with Harvard University. Together with researchers from several parts of the world, he has examined how the fungus has moved between continents and which genetic lineages have travelled with it. Using samples from multiple regions, he has followed the fungus’ genetic fingerprint.

The data reveal that the disease does not move as a single advancing front but as several separate lineages that have taken different routes out of North America.

The result is a map of the spread that can be used for more targeted monitoring of new outbreaks, the researcher says.

“From a research standpoint, the study shows that this global expansion involves multiple genetic lineages, which we can now begin to monitor more precisely,” Michael Bradshaw points out and continues: “In a broader perspective, this is a reminder of how quickly plant pathogenic fungi can move and how important it is to detect them early, before they become a permanent problem in production.”

Plant diseases tend to spread in two ways. Either the pathogen travels with a global crop into new regions, or the crop itself meets local pathogens that jump over from related wild plants. The new study shows what happens in the first case – and how quickly it can unfold.

An enquiry set the search in motion

The story began with an enquiry from a large berry company in the blueberry sector. The company contacted Michael Bradshaw after finding powdery mildew on blueberries and needing help in identifying it. Along the way, it became clear that similar powdery mildew outbreaks were appearing in several widely separated production areas.

“For a plant pathologist, this is the kind of information that sets off alarm bells. There was a gap between what the literature said and what people were actually seeing in the field. On paper, the disease looked more limited. But out in production, a different pattern was taking shape – one that suggested that the pathogen was moving with the crop itself.”

The fungus already seemed to be appearing across distant production regions.

This set in motion a project that combined modern sequencing with old herbarium collections. In total, 173 samples were included – both fresh finds and historical specimens, some collected more than 150 years ago.

By comparing the fungi’s DNA across time and geography, the researchers began to see patterns that would otherwise have remained hidden.

The work was anything but straightforward. Powdery mildew fungi cannot simply be grown in the laboratory, and they are often difficult to tell apart, both visually and genetically. That is why the old specimens mattered so much. They enabled the researchers to ask whether today’s outbreaks represent something new or whether older lineages have simply gained new opportunities to spread as blueberry cultivation has become global.

“The key was gaining access to material from multiple regions, so we could compare broadly and systematically. They were remarkably helpful and made it possible to collect and sequence material across continents,” says Michael Bradshaw.

Two tracks trace the disease’s journey

When the researchers assembled the pieces, a clear pattern emerged. They identified 50 genetic variants and found signs of two primary introductions to other parts of the world. One lineage appears to have reached China, Mexico and California. The other appears in Morocco, Peru and Portugal.

“We see much higher genetic variation in eastern North America than elsewhere – a classic signature of a place of origin. The fungus has had a long time to diversify there. Outside the region, the genetic trace is much narrower, which is what you would expect after more recent introductions.”

For Bradshaw, the results are first and foremost key to understanding how the disease moved from a likely origin in eastern North America into new production regions – following the crop as it spread globally.

This kind of mapping, he says, makes it possible to speak more precisely about routes of spread and risk instead of guessing.

Out in the field, it carries a different weight. “The real impacts are more subtle but economically meaningful: reduced yields, increased management costs and greater reliance on fungicides once the pathogen becomes established,” says Michael Bradshaw.

A subtle disease with major consequences

The researchers estimate that control can cost between USD 200 and 2,250 per hectare. Set against the global area planted with highbush blueberries, that points to a potential bill running into very large sums. That risk helps explain why the disease causes concern – even though it can begin so quietly it looks like nothing more than dust.

The finding lifts the study beyond blueberry fields and shows the difficulty of containing a plant disease once it has become part of production. The fungi are microscopic, spread efficiently and can travel far by wind, by plants and through the movements built into production itself. Once they have gained a foothold, they are notoriously hard to remove again.

“That is where the results become very concrete. They can be used for targeted monitoring in areas where the disease has not been recorded yet, and they can support breeding work, in which understanding multiple genetic lineages is important for developing more durable resistance. So this is not just a reconstruction of the past – it is also a tool for acting earlier next time.”

The next riddle is host switching

Even though the researchers now have a much clearer picture of the disease’s journey, a central question remains open: why some fungal lineages are so tightly tied to particular hosts, whereas others can suddenly jump to a new plant species.

“That question goes far beyond blueberries. Closely related powdery mildew fungi can be almost impossible to distinguish genetically and morphologically, yet they behave very differently biologically. Some stay strictly with one host, whereas others find a way into a new one. We still do not know which genetic, molecular or ecological mechanisms determine that.”

Bradshaw therefore sees the system as a rare opportunity to study host switching while the fine evolutionary traces are still visible. The work also continues beyond blueberries. He and the researchers around him are already working on a new study of strawberries and raspberries, in which the pattern looks different.

In blueberries, the story looks like a fungus that is likely native to the East Coast of the United States and is now spreading with a global crop out into the world. For strawberries and raspberries, the pattern is different: crops are moved into new regions and meet local pathogens that jump over from wild relatives.

“Taken together, the two systems point to something larger. In blueberries, we see a pathogen that follows the crop out into the world, while in strawberries and raspberries we see crops meeting local pathogens that jump over from wild relatives,” says Michael Bradshaw.