Farmers encouraged to plant varieties resistant to cyst nematodes, SDS to avoid yield losses.

This spring’s cool, wet weather is making difficult for farmers to plant their soybeans and making it easier for a well-known pest to affect this year’s crop.

Sudden Death Syndrome (SDS), a soil-borne fungus, is a disease that is highly dependent on the weather and the resistance level of the planted soybean varieties. Dr. Leonor Leandro, Iowa State University assistant professor of plant pathology, says conditions favoring SDS include compacted soils, soils with poor drainage and the interaction with the soybean cyst nematode.

Fields with a history SDS are also more at risk.

Leandro says years with abundant rainfall throughout the season will be most favorable for SDS.

“This year, we’ve had a particularly cool spring and planting has occurred later than usual,” Leandro explains. “According to the National Agricultural Statistic Service, only 16% of soybean acres in Iowa had been planted by May 19, which means we’re planting a majority of our soybeans after May 20.

"In this last part of May, we’ve had soil temperatures in the mid-to-upper 60’s, and this is going to give the season an advantage to get out of the ground and escape infection. We may still see some infection of the roots, but it won’t be as severe as if we had planted in cool soils.”

Leandro says the unknown factor is how wet the remaining season will be. If the current prediction is correct and the summer will be dry, the risk of SDS will be low. But if the soybean plants get into the reproductive stages and good rainfalls occur, Leandro says SDS might show up.

She says soybean yield losses vary greatly, depending on the weather in a given year or the location.

“Up to 100% yield losses have been reported in particular fields or sections of the fields. This especially occurs when the disease shows up in early reproduction stages," he says. "In 2010, Iowa experienced a very widespread, severe epidemic, and that year, the loss was estimated about 28 million bushels. In 2011 and 2012, the losses were very little because the summers were so dry.”

Leandro says the important thing for growers to note is that while the weather each year cannot be predicted, measures can be taken to reduce the impact of SDS if it appears. The most important measure is to plant SDS-resistant soybean varieties.

“There is no complete resistance to SDS, so any variety will develop some disease if it is in very favorable conditions, but they are going to suffer less yield loss than the more susceptible varieties,” she adds. “Many of the varieties that are resistant to SDS are also resistant to the soybean cyst nematode.

"Growers should try to select varieties that have resistance to both because there is an interaction with the soybean cyst nematode and SDS. If you manage both at the same time, you are going to be more effective at reducing losses from SDS.”

In addition to using resistant varieties, Leandro suggests growers use an integrated approach of other disease management practices to minimize yield losses. For SDS, that would include reducing soil compaction, avoiding planting in cool, wet soils and improving drainage.

It’s also important to use good farming practices that maintain healthy soils with good fertility levels, good structure and good biological activity because a healthy soil helps plants develop a vigorous root systems, therefore, having a better defense against pathogens.”

To learn more, visit the Iowa Soybean Association (ISA) Production Research website, www.iasoybeans.com/productionresearch, and click on the podcast, “Protect Soybeans from SDS Unknowns.”