Wednesday, August 5, 2009
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
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
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.