New potentially damaging strawberry virus raises questions, concerns

By Doreen Muzzi

An unwelcome visitor from Europe strawberry latent ringspot virus has been discovered on strawberry plant samples from California and British Columbia. While researchers don 't think it will become a problem for U.S. growers, they say it shouldn't be ignored, either.

I suspect it is present in every state, and I suspect it has been here for a long time, says Bob Martin, a plant pathologist with the Agricultural Research Service Horticultural Crops Research Unit in Corvallis, Ore. Martin is leading the agency 's effort in studying and preventing the virus, with part of his work focused on the disease 's presence in California.

We probably don 't have very efficient vectors here for the virus, so it probably won 't become a problem, he says. But we do need to be aware of it for those people running certification programs and quarantine programs. It was never included in our screening before, because we didn 't know it occurred in the United States.

The virus, which has been a problem in Europe for the past 30 to 40 years, was found recently on 17 percent of California strawberry samples and 4 percent of British Columbia strawberries. The virus also has been found in a variegated mint variety.

Adib Rowhani, a University of California, Davis, plant pathologist, screens all foundation mother plants and some of UCD 's breeder advanced selections for the presence of strawberry latent ringspot virus and other diseases.

Until this year, the tests used for detecting viruses did not differentiate among the different types of viruses infecting strawberry plants. Plants simply tested positive or negative for the presence of one or a combination of several different viruses.

Although we've had plants test positive using this screening method, we couldn't determine which specific virus was causing the positive result, Rowhini says.

This spring Rowhani and his colleagues began using a new molecular-based test, which screens specifically for strawberry latent ringspot virus. Since they began using the new test, no foundation stock has tested positive for the virus.

If one were to test positive, it would immediately be removed from the foundation program, Rowhani says.

A virus among many

Scientists with ARS, Oregon State University and Elmhirst Diagnostics of British Columbia discovered the virus by doing a broad-spectrum test to look for viruses that may be involved in strawberry decline and the variegation of mint. They compared nucleic acid and protein sequences of the virus from strawberry and mint to those in databases.

We were looking at declining plants all along the West Coast, and we started looking for viruses to explain it, Martin says. We then developed lab tests for all known strawberry viruses, and that 's how we picked it up. Shortly after that, we found it in variegated mint.

Martin doesn 't think the virus will have a big impact on growers, but certification programs should be aware of it, he says.

If the certification programs pick it up and deal with it, it shouldn't become a problem for growers.

A minty relationship

In ornamental mint with yellow veins, the veins actually are symptoms of three viruses, including strawberry latent ring-spot, so the disease probably has been in the United States for many years, he says. Ironically, the bright yellow vein is what makes the mint variety so popular as an ornamental plant.

Mint infected with the virus was found in Maryland, Nebraska, Ohio, Texas and Oregon.

As for control of the virus, Martin doesn't think growers should be concerned specifically about strawberry latent ringspot.

The standard methods growers use for controlling other viruses will adequately control this virus, so they shouldn 't worry about any new control methods.

If states adequately address the issue by adding strawberry latent ringspot virus to the viruses they test for in foundation material it shouldn't become a problem for growers, Martin says.

Several unknowns surround virus

Researchers aren't certain about what the virus can do on it's own because it always has been seen with other viruses, such as strawberry crinkle, strawberry veinbanding and strawberry mild yellow edge, he says.

Normally, whenever we see the virus, it is with other diseases, Martin says. In these instances, the plants are stunted and the yield can be cut to zero. Unfortunately, we haven't seen the virus by itself, so we can't say for certain what damage it can cause to plants.

Because the California and British Columbia strawberry plants tested positive for five different viruses, scientists say it is difficult to determine how much yield loss can be attributed to each individual disease.

The virus has been found where plants were dying or not growing well and where scientists were testing for other diseases, Martin says.

Strawberry latent ringspot is reportedly transmitted by nematodes. But most California strawberries are planted in fumigated soil.

The search for a vector

In Europe, it is reported that the virus is transmitted by nematodes, Martin says. We have no reason to doubt that that is the same case here. But based on the sequence, it is a very unusual nematode-transmitted virus.

Many chemicals that have been used to control viruses transmitted by nematodes currently are in the process of being pulled from the market due to environmental concerns.

So Martin and other researchers are looking for alternative ways of controlling nematode-transmitted diseases. One solution may be rotating to a crop that is not a virus host so the nematodes will lose the disease and no longer be able to spread it.

Strawberry latent ringspot so far has been found only on strawberries and mint in the United States, but scientists say it can infect many broadleaf crops, including roses, rhubarb and raspberries.