COLUMBUS – Although more than 200 new soybean varieties that resist soybean cyst nematode have been made available so far for the 2003 season, growers shouldn’t shirk monitoring populations in their fields.
Of the new resistant varieties, all but four originate from the same resistant source.
This paints a picture that tells Ohio State University plant pathologist Anne Dorrance that eventually resistant varieties from PI88788 will no longer be effective against the pest.
Increased importance. And, the amount of time it takes to identify and incorporate resistance genes from new sources of resistance, as well as breed a high-yielding competitive soybean variety, increases the importance of continued testing despite the varieties they plant.
“The list of new varieties coming from PI88788 is telling us that growers need to be sampling their fields and monitoring SCN populations often to make sure that the source of resistance they are planting is still working,” Dorrance said.
“If the grower plants the same resistant variety year after year, eventually the nematode will adapt to it and the line will no longer be effective.”
Crumbling defense. Dorrance said the barrier between soybean cyst nematode and certain resistant soybean varieties is already crumbling in limited areas through Ohio.
Dorrance said this will likely continue as the nematode populations adapt to varieties that carry the PI88788 resistance.
Incomplete. “I think what growers don’t understand is that resistance does not mean a complete resistance,” she said
“The way resistance is measured to soybean cyst nematode is by how many females reproduce compared to the susceptible check.
“There will still be females reproducing on that resistant line, developing nematodes that eventually will develop an appetite for that particular resistant variety.”
Testing for eggs. The best way for growers to determine whether they should plant a susceptible soybean variety, a resistant variety or a non-host crop is to test their soil for egg populations.
“This may sound ludicrous, but growers should be planting susceptible soybean varieties if the populations are from zero to 40 eggs per cup of soil,” Dorrance said.
“By selecting a susceptible variety, it keeps the genetic pool in check so that all nematodes are competing with each other for food and no one nematode with a particular appetite for a specific source of resistance can dominate.
“If a grower breaks even on nematode egg populations compared to a previous egg count after planting a SCN-resistant variety, it may mean that the nematodes are able to maintain their populations under the soybean variety being planted, eventually increasing in numbers at the same rate as if a susceptible variety had been planted.
“Under this situation, growers should either switch to a non-host crop or switch to a different resistant line, avoiding varieties that share similar genetics to the line that the grower was planting.
“Seed companies should be able to help growers pick a different variety.”
Looking ahead. Though PI88788 has been the mainstay of soybean cyst resistant lines for Ohio growers for the past decade, Dorrance said growers should be keeping their eye out for the development of new lines that carry resistance.
Some examples are Peking and Hartwig.
“Though we’ve got new varieties coming down the line, we are still very limited at this point as to what is available for growers in terms of different resistant lines,” Dorrance said.
Testing importance. “That’s why it’s important to test fields for the presence of SCN. A grower does not want to waste a good source of resistance by overusing it.
“If this happens, it could mean losing the crop,” Dorrance said.
Additional information on soybean cyst nematode, including a list of resistant soybean varieties for the 2003 growing season, can be found at http://ipm.osu.edu.
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