Blueberry growers tending the rapidly growing acreage of Eastern Washington generally enjoy low disease and pest pressure. However, that doesn’t mean they get a free pass.
Viruses can attack plants in their area, too. And they have.
Washington State University researchers know of one Eastern Washington blueberry field infected with tobacco ringspot virus, and they don’t know for sure where it came from.
“This is a kind of wake-up call,” said Naidu Rayapati, a virologist at WSU’s Irrigated Agriculture Research and Extension Center in Prosser.
Tobacco ringspot is a virus common to a wide array of crops, including Washington’s wine grape and tree fruit industries. However, the 2015 finding is the first documented blueberry case on the West Coast, Rayapati said.
The Pacific Northwest blueberry industry is more familiar with scorch and shock viruses. Those are present on the west side of the Cascade mountains, where the industry has a longer history, though growers in Western Washington are not currently removing any bushes due to those viruses, said Lisa DeVetter, associate professor of small fruits horticulture at WSU’s Northwestern Washington Research and Extension Center in Mount Vernon.
Rayapati suspects Eastern Washington, where acreage has been surging the past 10 to 15 years, most likely has scorch and shock as well, but problems probably are too minor to notice, he said. So far, at least. Given time, diseases can become major economic problems — the way little cherry disease, or X disease, has walloped the state’s sweet cherry industry.
“This is an example of when industry stakeholders have to pay attention,” Rayapati said.
The discovery of a new blueberry virus in Eastern Washington does not surprise Jack Ringe, an agronomist for OVS, an agricultural supply vendor. New industries usually start squeaky clean. As they grow, more plant material moves around and, odds are, eventually a disease or two takes hold, he said.
Rayapati and his team diagnosed the Washington tobacco ringspot case with next-generation sequencing technology after a grower with unexplainable decline symptoms asked for help. He had recently planted the field on virgin ground, so that ruled out replant problems.
Rayapati suspects the virus, transmitted by soilborne dagger nematodes, came with some of the bushes, purchased in pots from a nursery. He and the grower found nematodes in the soil. Since then, the virus has slowly been spreading and the grower has patches of decline, though it has not turned up in any other fields.
Symptoms of tobacco ringspot in blueberry typically manifest strikingly around bloom, with small, missing or deformed leaves, stunted plant growth, a lack of flowering and, later, small berry clusters that don’t mature. Leaves also bear ring-shaped lesions — for which the virus is named.
Blueberry leaves sickened by scorch virus, which is transmitted by aphids, can have similar-looking leaf patterns, while the pollen-borne shock virus might not show any symptoms at all. Knowing the difference informs management decisions.
“It’s very important to have a proper diagnosis,” Rayapati said.
Viruses have no cure. The best — and most common — advice is to avoid them by only purchasing and planting certified plants. Most nurseries have rigorous protocols to test for viruses, including tobacco ringspot in blueberries, said Gwen Hoheisel, a WSU extension specialist.
Nothing is foolproof, of course. The virus has a wide host range and can survive dormant in native plants and weeds.
“All kinds of stuff has the ability to host nematodes and viruses,” she said.
Fumigation before planting is a preventative tool, but once plants are in the ground, options are limited, she said.
Within the row, consider placing physical barriers of wood, plastic or metal between infected plants and their neighbors, to stop nematode movement, then remove the infected plant, she said. Researchers are still studying how well that works.
Between rows, maintain cover crops inhospitable to nematodes or the virus. Researchers, such as Inga Zasada of the U.S. Department of Agriculture in Corvallis, Oregon, are researching how different species of nematodes interact with cover crops and other plants in the Pacific Northwest.
—by Ross Courtney
Managing blueberry diseases in Michigan
Michigan State University small fruit and hop pathologist Timothy Miles would like to see blueberry growers mix more biological products into their spray programs. Biologicals might not be as powerful as the “big guns” — conventional products growers are used to using — but overreliance on a few powerful tools can lead to pesticide resistance.
Diversifying spray programs costs more and complicates management, but in the long run doing so offers the best strategy for effective pest management, he said.
Miles spoke during the Great Lakes Fruit, Vegetable and Farm Market EXPO in December. He discussed evolving management strategies for blueberry fruit rots, especially anthracnose, the top disease problem for Michigan blueberry growers.
Anthracnose fruit rot can harm yield, shelf life and fruit quality, and it can lead to unacceptable microbial levels in the field. Infections happen early, around bloom, and the pathogen reproduces quickly. Infected fruit fall off the bush, putting a massive amount of inoculum in the field. A single infected berry can contain 10 billion spores, he said.
Anthracnose control accounts for nearly half of the fungicides used in Michigan blueberries. The backbone of most spray programs includes Fungicide Resistance Action Committee (FRAC) groups 11, 9 and 12. Anthracnose has developed resistance to FRAC 11 fungicides, but not yet to other modes of action. Miles recommended a few strategies to manage fungicide resistance, including applying half rates, rotating FRAC groups, targeting sprays around bloom and using dormant sprays.
As for biological products, he said quite a few have proven to be effective in spray programs with other effective products, including Stargus (Bacillus amyloliquefaciens), Howler (Pseudomonas chlororaphis) and LifeGard (Bacillus mycoides isolate J).
Cultural practices help, too. These include pruning, improved weed control, adjusting overhead irrigation timing to minimize wetness and using drip irrigation. When the berries are ripe, pick them quickly, keep the fruit cool and don’t harvest when it is raining. Cultural practices mostly come down to keeping the plants as dry as possible, Miles said.
More long-term practices include wider plant spacing and planting resistant varieties. But even resistant cultivars, such as Elliott, can develop shoot infections. If resistant cultivars are planted close to susceptible cultivars, both should be protected with fungicides and pruned regularly, he said.
Miles also discussed alternaria fruit rot, considered to be more of a storage or spoilage disease. In postharvest situations, infections often occur on the stem scar, or calyx end, and quickly spread over the entire berry. To manage Alternaria, harvest quickly, keep fruit cool, don’t harvest when it is raining or when the fruit is wet, and use a preventive spray program, he said.
Effective fungicides for Alternaria include FRAC groups 7, 9, 11 and 12, Miles said.