Increasing Stresses Could Impact Soybean Yields

July 15, 2002

WOOSTER, Ohio - Ohio's soybean crop may be hard-pressed to deliver this growing season.

Some fields, already suffering from little or no rain, are also being attacked by soybean cyst nematode. The combination of the two stresses could impact yields.

Anne Dorrance, an Ohio State University plant pathologist with the Ohio Agricultural Research and Development Center, said that soybean cyst nematodes, found in some soybean fields every year, are showing up earlier in the season in such high numbers that they are beginning to kill plants.

"We are finding females with hard cases much earlier than we would in a normal growing season," said Dorrance. "In the samples we got before the 4th of July holiday, we were easily finding 200 females on plants that were only in their third trifoliate." With each female potentially laying 250 eggs, the economic loss may be dramatic. Over 100,000 eggs per cup of soil easily translate into yield losses ranging anywhere from 20-50 bushels per acre depending on the variety and soil type.

Trying to make lemonade out of a lemon of a season, Dorrance is encouraging growers to take advantage of the drought conditions to scout fields that may be affected by soybean cyst nematode.

"The drought is just accentuating symptoms that we wouldn't normally see. You can have high populations and never see an above-ground plant symptom in a normal year," said Dorrance. "This year, because of the drought, we are seeing severe symptoms - mainly stunted plant growth - in fields where soybean cyst nematode is present. So that's a good opportunity for growers to get out there and identify those problem fields." She said the best way to determine the presence of female soybean cyst nematodes is to carefully dig up the plants, gently shake the roots and look for the females on the roots. They will appear as tiny white "pearls." Notes should be taken and those fields sampled in the fall for egg counts through laboratory tests.

"A good accurate egg count in the fall is the first step in soybean cyst nematode management," said Dorrance. "The best management practice is crop rotation, crop rotation, crop rotation. Once the insect is present, you can't get rid of it. You just have to keep monitoring it so it's always below economic thresholds, and during the growing season, it's important to identify those fields that have soybean cyst nematode." Yield loss threshold begins at 200 eggs per cup of soil. At 2,000 eggs per cup of soil (equivalent to about 10 females), most soybean varieties susceptible to soybean cyst nematode will be damaged to the point where it's uneconomical to grow them. At this level farmers are encouraged to plant resistant varieties or rotate to a non-host crop.

Farmers with egg mass numbers between 2,000-5,000 per cup of soil should rotate to a non-host crop the next growing season, then return to resistant soybean varieties the following year. At 5,000 eggs per cup of soil farmers should avoid growing soybeans altogether and switch to a non-host crop for two to three years.

Dorrance said fields that are having the most soybean cyst nematode problems this growing season are the ones with high populations of winter annual weeds.

"In fields that have good weed control, it takes a good 30 days from planting before you begin to see females. We usually tell farmers to scout their fields the end of July or early August," said Dorrance. "But this year, with the winter annuals, those nematodes were immediately ready to develop females. We are finding brown cysts, from where the eggs hatch, which is highly unusual." Additionally, smaller root systems from lack of rain have created stress conditions for the plants, also making the increased presence of soybean cyst nematode more of a problem.

"There aren't a lot of roots on some of these plants, so you have a higher concentration of cysts on some very young plants," said Dorrance. "The combination of drought stress and soybean cyst nematode is really doing some damage to the soybean crop. Some areas of Ohio are going on four weeks without any rain and it's more than some of these beans can handle."

Author(s): 
Candace Pollock
Source(s): 
Anne Dorrance