Will the pests that caused so much trouble for berry growers be back in 2014?

There is good news and bad news on that subject. The two viruses that together took a big chunk of the strawberries in Virginia and the Carolinas are unlikely to be a significant problem again.

But the fruit fly spotted wing drosophila (SWD) is an issue that appears to be here to stay, said Hannah Burrack, North Carolina State University Extension entomologist.

“It can be a problem on any soft-skinned fruit. In North Carolina, it has been found on strawberries, blueberries, blackberries, raspberries and cherries. In some nearby states, it has also been found on peaches and grapes. SWD preys on soft and sweet fruits, and if you are growing any, you need to be prepared to manage it.”

In the South, SWD was first spotted in Florida in 2009, in Mississippi and both the Carolinas in 2010, and most other Southern states in 2011 and 2012. It is now in 41 states.

“Everywhere it is found, it is likely to require management, and you need a plan to manage it,” said Burrack. “There aren’t a lot of tools for dealing with SWD, so we need to optimize the tools we have.”

Here are some tips to get the best control:

  • Harvest as frequently as possible and get as good post-harvest quality control as you can get.
  • Keep the fruit cold after your harvest it. “Immature SWD don’t develop in temperatures colder than 41 degrees F, although they may not actually die,” she said.
  • Take steps to make sure the canopy is open.
  • Use insecticides if needed. One new insecticide may be available for use on blueberries this season, Burrack said, but she can't yet reveal the name.
  • Don’t let any berries stay on the ground after harvest, since the insect can use them to reproduce.
  • It may be less important to aggressively protect strawberries during the first weeks of harvest. SWD are likely to be most significant in strawberries later in the season.
  • SWD are also likely to be a bigger issue in day-neutral strawberries. “That is one of the crops where we found the first reproducing population of SWD in North Carolina in 2010,” Burrack said.
  • SWD populations may be higher as they move into later crops, such as rabbiteye blueberries, blackberries and raspberries.

Double virus surprise, whammy

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.