Nematode “Cleansing” Technique May Benefit Potato Crops

Rotating potatoes with alfalfa or spearmint may offer farmers a sustainable way to reclaim crop soils contaminated with corky ringspot disease, Agricultural Research Service studies suggest.

Corky ringspot--a brown, bull’s-eye blemish on tubers--is caused by the tobacco rattle virus, which is passed into potato plants by wormlike soil organisms called stubby root nematodes. In affected areas such as Washington’s Columbia Basin, where 5,000 acres are contaminated, potato farmers have either stopped growing the crop altogether or resorted to fumigating soil to kill nematodes that spread the virus.

At $250 per acre, however, fumigation is expensive. Plus, it kills soil organisms other than nematodes. A more sustainable, pest-specific approach could come from rotating potatoes with alfalfa or spearmint to rid the nematodes of their viral payload, according to scientists Rick Boydston, Pete Thomas and Hassan Mojtahedi. Boydston and Thomas are at ARS’ Vegetable and Forage Crop Production Research Unit in Prosser, Wash. Mojtahedi works for Washington State University.

Their strategy is based on two observations: First, the virus can’t survive in alfalfa or spearmint, so the nematodes can’t acquire it while feeding on the plants’ roots. Second, the nematodes naturally shed the virus from their bodies by molting. Given enough time, the scientists reasoned, nematodes that fed only on these plants eventually would rid themselves of the virus, and thus pose little or no danger of infecting a subsequent potato crop.

Indeed, in greenhouse trials, virus-bearing nematodes that fed on potted alfalfa or spearmint plants for three months lost their ability to infect disease-free potato plants. Conversely, nematodes that fed on tobacco, a natural host of the virus, retained their ability to infect disease-free potato plants.

Some farmers already practice this rotation but still fumigate fields with a previous history of corky ringspot. The key, scientists emphasize, is to eliminate weeds in alfalfa or spearmint crops where nematodes can acquire the virus. These include hairy and black nightshade, downy brome, common chickweed and prickly lettuce. They are among 38 weed species the scientists have tested.

ARS is the U.S. Department of Agriculture's chief scientific research agency.