In the study, Bradshaw and his colleagues examined historic and modern plant leaves plagued by powdery mildew. The collection includes 173 samples from North America, Europe, Africa, and Asia; one sample analyzed from a North American herbarium was collected over 150 years ago, while the foreign samples were all collected within the past five years. In this study, powdery mildew was first spotted outside North America on a farm in Portugal in 2012, as noted by a co-author of Bradshaw's who was employed by a major berry company.
The study showed that the disease originated in the eastern United States and was set loose globally in two different introductions. One strain of E. vaccinii found its way to China, Mexico, and California, while a different strain wound up in Morocco, Peru, and Portugal. Bradshaw thinks humans are responsible for the spread as nursery plants traveled to foreign shores.
Interestingly, the study also showed that the E. vaccinii fungus found in blueberries in other countries appears to solely reproduce asexually; both sexual versions of the fungus are not required in reproduction, whereas the fungus reproduces sexually and asexually in the United States.
The study also worked with a large company and farmers to provide an estimate of the global cost of powdery mildew to blueberries, reflecting the cost of spraying fungicide to prevent or reduce powdery mildew. The study estimates a cost range of between $47 million and $530 million annually for the global blueberry industry.
Finally, the study provides some early warning signals to important blueberry-producing areas, like the U.S. Pacific Northwest. Conditions there are ripe for powdery mildew to take hold and spread, but the disease has not found its way there yet.
"Disease spread could also be impacted by agricultural conditions," Bradshaw said. "Some areas that grow blueberries in tunnels, or enclosed areas, seem to have worse disease outcomes than areas that grow blueberries outdoors without any covering, like in North Carolina," Bradshaw added that the researchers utilized a useful tool that can help identify E. vaccinii strains to aid farmers and other researchers.
"It's difficult to identify the fungus that causes powdery mildew in blueberries, so we input our data in a public database developed at NC State by a co-author, Ignazio Carbone. This platform allows growers to enter their data and learn which specific strain is in their fields," Bradshaw said. "That's important because understanding the genetics can warn farmers about which strain they have, whether it is resistant to fungicides, and how the disease is spreading, as well as the virulence of particular strains."
For more information:
Michael Bradshaw
NSU
Tel: 1 (919) 515-5691
Email: [email protected]
www.news.ncsu.ed