Strawberry Viruses Could Reduce Crop Yield

6/8/2013 7:00 AM
By Ann Wilmer Maryland Correspondent

Although the late harvest of strawberries can be blamed on the weather, the size of the harvest has been affected by a virus.

Michael J. Newell, faculty research assistant and program manager for horticultural crops at the Wye Research and Education Center, said agricultural researchers noted a problem with strawberry plants in the states last fall. Sometime last November, they started getting reports that strawberry plants were not growing well and they began to investigate, not suspecting a virus at the time.

“Most of the plants we get for plastic culture — planted beneath black plastic in the fall to overwinter — come from plant nurseries in Canada and Nova Scotia,” Newell explained. The strawberry plants produce runners or “daughter” plants that they harvest and send them to plug-producers who put them in cell packs and nurture them for four weeks to produce a “plug plant.”

These are planted by farmers on plastic in the strawberry fields. Delmarva farmers like Canadian plants because several soil diseases that affect this area do not do well that far north.

Strawberry specialists visited growers in Canada and collected samples of plants that were not growing well. They were sent to Dr. Bob Martin, a USDA Agricultural Research Service, or ARS, small fruit virologist at Oregon State University specializing in small fruits such as strawberries. About half of the plants Martin tested were infected by two viruses: strawberry mild yellow edge virus and strawberry model virus.

Newell said that one virus doesn’t usually affect plants, and growers will see no symptoms. When plants are affected by multiple viruses — called a virus complex — you do see symptoms. Researchers started tracing back the origin of the plants and identified a nursery in the Great Valley area of Nova Scotia that had the virus complex.

U.S. plant nurseries in California and Oregon have adopted stringent controls to keep plants disease free, but Canada does not have such an astute program. Because the U.S. is a big market, Canadian producers are changing their protocols and instituting a clean plant program.

The viruses are transmitted from plant to plant by certain aphids. Infected fields are being isolated and growers are doing intense aphid watching.

Newell said researchers hope the protocol that has been instituted means they we will never see the viruses again.

Affected plants are smaller in size. Newell said it’s rare that anyone had a complete field of virus-infected plants. Even those infected with the viruses will produce strawberries that are safe to eat. However, infected plants that suffer another stress, such as inhospitable weather, go downhill fast. Farmers may suffer a 30 percent reduction in yield.

“It’s a loss but not a complete loss as long as you continued to take care of the plants,” said Newell. “But 30 percent could make a difference between break-even and a profit.”

The average production of a strawberry plant in the field is 1.2 pounds. Last year, area farmers enjoyed a record harvest of 2 pounds per plant. Newell said that spring plants ground in the ground have not been effected.

Strawberries grown in plastic-covered beds are planted in September. They can overwinter so farmers rely on mild fall temperatures to get nice growth out of the plant in the winter to advance the crop and make for an earlier harvest next year.

Drip irrigation is a more efficient use of water, and fertilizers and pesticides can be put in through the water. The pre-harvest interval for some of these chemicals can be as low as four hours but others must be applied earlier. Plastic allows better air circulation around the plant to reduce chance of disease and keeps plants from becoming waterlogged which can seriously impact fruit quality.

Historically, some of these plants are carried over and produce a second-year crop. Experts are recommending against that practice now and encouraging heightened scouting for aphids. Instead, they recommend that the plants be destroyed with a chemical herbicide, the plastic and drip line removed and then strawberry plants plowed under.

For more information, email Newell at or 410-827-7388.

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