One of the most memorable events of the strawberry season last year came early when two viruses arrived in Virginia and the Carolinas at the same time.

They had been found occasionally in this country before but separately and never side by side. They proved to be more damaging in tandem than either ever was by itself.

The two viruses--strawberry mottle virus and strawberry mild yellow edge virus―had come to the U.S. on infected plants that originated in a nursery in Nova Scotia. These plants were distributed throughout the U.S.

Frank Louws, North Carolina State University Extension plant pathologist, doesn’t know how much of a yield loss the viruses caused last season.

“It is hard to measure the impact of the viruses within an infected planting,” he said. “The effect on the overall crop is going to be fairly minimal–maybe four to five percent reduction in overall production for North Carolina in 2013.”

Symptoms of the viruses include poor plant growth in spots or in entire fields, older leaves turning bright red, yellowing along plant edges or on emerging leaves and dead plant tissue along leaf margins.

Common aphids can transmit these viruses to healthy plants and may be hosted overwinter on the weed lambsquarter.

But there is good news: if growers start next season with all-new plants, there should be no residual problem, said Louws. “I think there is a very low risk of re-infestation. I certainly don’t expect it to persist on North Carolina farms.”

For the coming season, said Louws, a farmer’s goal should be: “Question your suppliers, and make sure you get good healthy plants.”

There is one cultural practice that growers should avoid: Leaving potentially infected plants in the field over the summer. “It risks virus spread into next year's crop,” said Charles Johnson, Virginia Tech University Extension plant pathologist.

The origin of the 2013 infection is at present unknown.

“These viruses can reduce yield as much as 30 percent in a severe situation,” said Johnson. “The losses may be worse with bare root plants than plug plants.”

There is no human health risk with these viruses, he added. They affect plant growth and productivity but pose no threat to consumers.