Jul 28, 2009

Michigan Moves to Stop Two Exotic Blueberry Plant Diseases

Two exotic blueberry plant diseases have been identified in Michigan for the first time — blueberry shock and blueberry scorch. Both viral plant diseases have the potential to cause significant losses to blueberry producers, but they are not a threat to food safety or human health.

The Michigan Department of Agriculture (MDA) and Michigan State University (MSU) officials are implementing a response plan to eradicate these diseases. Intensive testing is going on within a 2-mile radius of the three sites where the diseases were found.

According to MSU plant pathologist Annemiek Schilder, shock was found at the Trevor Nichols Research Complex near Fennville by MSU scientists who noticed symptoms and conducted tests to identify the cause.

Scorch was found on two commercial farms in southwest Michigan during routine testing conducted by MDA.

According to MDA Director Don Koivisto, the MDA Pesticide and Plant Pest Management Division had been watching for these diseases, which are endemic in certain West Coast and mid-Atlantic states, and was taking steps to prevent them from entering Michigan. MDA issued an external quarantine in 2002 and conducted routine sampling.

“Although not totally unexpected, we are very disappointed these two serious diseases have been identified in southwest Michigan – the state’s blueberry belt,” Koivisto said.

Michigan is the nation’s largest cultivated blueberry producer, with more than 19,000 acres producing 110 million pounds of blueberries valued at $124 million last year.

“MDA and its partners are working in concert to quickly eradicate this threat and safeguard this valuable commodity. These viruses do not pose a food safety or health risk, and consumers can select Michigan blueberries with confidence and pride in the product,” Koivisto said.

Schilder was part of the team that identified blueberry shock in a research plot at Trevor Nichols Research Complex.

“Blueberry shock symptoms may look similar to spring frost injury or common plant diseases such as Phomopsis twig blight, but ELISA testing confirmed our suspicions,” Schilder said. “Researchers, regulators and growers have learned about blueberry shock and scorch from Web sites and books, but having it here will help us all learn more. We’re going to make the best of this situation by researching optimal testing protocols before the plants are destroyed.”

She estimates that about 50 plants at the station are infected, and that the infection occurred three or four years ago.

Blueberry scorch virus was discovered on two private farms in west Michigan. The infected plants have been destroyed and MDA agents are testing adjacent areas to verify blueberry scorch disease has not spread. About 40 farms were tested, with no infected plants found. The plantings were young, so this disease has only been here a year or less, Schilder said.

Blueberry shock-infected plants suffer from loss of foliage and blossoms, resulting in yield loss the first year of infection. Fruit production may resume.

“Shock-infected plants recover in the Pacific Northwest,” Schilder said. “Growers lose a few years of production but the disease runs its course.”

MSU researchers believe the disease could be more severe in Michigan’s northern climate than in its native region, the Pacific Northwest. Infected plants serve as a source of infection to other nearby plants, since blueberry shock is transmitted by pollinating insects.

Blueberry scorch disease symptoms are similar to shock symptoms. In the spring, shoot tips will die back, sometimes on just a few branches. The flowers may blight just as the earliest blossoms open. Scorch-infected plants repeat this symptom cycle each spring until the entire bush becomes infected, typically within three years. Fruit production and shoot growth are seriously reduced on scorch-infected plants. Scorch is transmitted from plant to plant by aphids.

There are no known cures for either of these plant diseases.

“We believe that both diseases are restricted to small areas and can be eradicated,” Schilder said. “But buying plants from a nursery selling virus-tested plants is the best way to prevent further infection, because both plant diseases can be spread via infected cuttings.”

For more information, visit the MSU blueberry shock Web site, www.blueberries.msu.edu/shock.htm, the MSU blueberry scorch Web site, www.blueberries.msu.edu/scorch.htm, or call Schilder in the MSU Department of Plant Pathology at 517-355-7539.