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College of Food, Agricultural, and Environmental Sciences


Choose Fields Wisely If Switching to Soybeans

May 29, 2002

WOOSTER, Ohio - Growers contemplating switching their acreage from corn to soybeans should carefully choose which fields to make the transition.

Anne Dorrance, an Ohio State University Extension plant pathologist with the Ohio Agricultural Research and Development Center, said fields with a history of insect and disease problems on soybeans should not be planted with the crop.

"Our concern right now is that growers are beginning to watch the calendar, make estimations on yields and will begin to switch those corn acres to soybeans. That's going to be a real problem for some producers in Ohio," said Dorrance. "Problem soybean fields are the ones that should be planted with corn no matter how late the corn goes in because planting soybeans will just create worse problems down the road." One example of a problem soybean field is one with a history of soybean cyst nematode, a problem that is effectively controlled through crop rotations.

"Soybean cyst nematode doesn't care what the weather conditions are. It's active all the time and it attacks the plants at any growth stage," said Dorrance.

Researchers have found that when a susceptible soybean variety is planted in a field with soybean cyst nematode, there is a 10-fold increase in pest populations. When growers meet their rotation of corn or wheat, there is a 50 percent decrease in cyst populations.

"There are more problems with soybean cyst nematode with continuous soybeans than with rotated crops," said Dorrance. "Growers hopefully know which fields are susceptible to soybean cyst nematode. Those are not the fields that should be taken out of rotation." Dorrance also added that a grower's field that is producing five to 10 bushels lower than a neighbor's could be a sign of a pathogen problem. "That's also probably not a field to choose to switch from corn to soybeans," she said.

Fields with a history of Phytophthora root rot, a severe disease that can affect yields, should also be maintained for corn planting.

"What happens when we lose that rotation is the pathogen that causes the disease just builds up in huge numbers," said Dorrance. "That's also a sign that the resistance genes are no longer effective in that field, which is what we are beginning to see more of." Growers are recommended to watch soybean cyst nematode rotation and choose a soybean variety that has a source of resistance, especially if they miss a rotation. "If a grower misses a rotation, make sure that field gets sampled next year for cyst nematode and find out what the populations are like," said Dorrance.

Because of soggy fields from several weeks of rainfall, very little of the state's soybean crop has been planted. According to the National Agricultural Statistics Service, only 7 percent of the crop has been planted, compared to 84 percent this same time last year.

Dorrance said wet fields are creating ideal conditions for disease development, specifically Pythium and Phytophthora root rots. "The environmental conditions just don't get any better than this for these two pathogens," she said.

Growers are encouraged to select soybean varieties that exhibit high levels of partial resistance to Phytophthora root rot. "Choose a variety with one or two RPS genes plus high levels of partial resistance, also called field, general or tolerance in some of the seed catalogues. And treat the seed," she said. "These soils are just primed and ready to go with these diseases." Ohio State specialists are encouraging growers to stick to planting corn as much as possible. "We've got enough soybean problems in the state. We don't need another couple million of soybean acres to compound those problems," said Dorrance. "It may be one of those years, however, where growers don't have much choice, so if we can provide them with the tools to make a better management decision, it might minimize their losses down the road."

Candace Pollock
Anne Dorrance