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Soil Conditions Don’t Affect Potato Early Dying Pesticide

When potato growers treat the soil in their fields for early dying disease, the pesticide they use doesn”t doesn”t always perform consistently. Now, a University of Wisconsin-Madison scientist has found that the soil temperature, soil moisture and depth of the application don”t explain those inconsistent results. Instead, the solution to pesticide efficiency may be to find a way to seal the top layer of soil, preventing the chemicals from escaping into the atmosphere, according to Doug Rouse, a plant pathologist with the College of Agricultural and Life Sciences.

Early dying disease is a chronic problem in Wisconsin, where growers treat about 40 percent of the state”s potato acreage each year. If farmers don”t treat infected fields, they can lose part of their crop, according to Rouse, who spoke recently at the UW-Extension and Wisconsin Potato and Vegetable Growers Association Conference in Stevens Point.

Metham sodium, or Vapam, the pesticide that growers use for early dying, is expensive, may change the soil community, and is a possible threat to the environment. Therefore, growers want to use as little as possible while keeping the disease in check, Rouse says.

Working at the College”s Hancock Agricultural Research Station and on commercial farms, Rouse studied how several soil variables, including moisture content, temperature, and the depth at which the fumigant is injected, affect how well metham sodium worked. “We found that it really doesn”t matter, just as long as they get the fumigant into the soil,” says Rouse.

However, data from the field suggest a reason the fumigant doesn”t work efficiently: the chemical moves to the surface of the soil, where it diffuses into the atmosphere. “The top layer of the soil usually doesn”t have a lethal level in it,” he says. “It doesn”t kill the pathogens well.” A computer model developed by Rouse and soil scientist Phil Barak adds weight to this result by predicting low concentrations of fumigant in the top several inches of soil.

The answer to maintaining a lethal level of fumigant may be to create a seal over the top of the soil, trapping the chemicals and giving them time to work. Next year, Rouse plans to study whether spraying plant starch on the fields after fumigating helps to lock in the pesticide.

“We know we can do better, but now we have to figure out how,” says Rouse. He will also investigate whether years of fumigation have produced organisms that can resist the pesticide, or break it down.

Rouse”s research is funded by the Wisconsin Potato and Vegetable Growers Association.