Impact of Weather on SCN Infestations Explored by SDSU Specialist

It’ll be a while before the extent of Soybean Cyst Nematode (SCN) issues is known. Among determining factors, South Dakota State University plant pathology specialist Dr. Madalyn Shires is keeping a close eye on the weather.

“While we have had a pretty wet and fairly cool spring, SCN is more affected by the heat and possible drought in the summer,” said Shires, “so we’re starting to watch those long-term forecasts to see if we’re forecast to have another year of drought or high temperatures.”

Soybean cyst nematode infestations are worse during hot and dry seasons, which the parasites like the best. Unfortunately, even the wettest season wouldn’t result in a year free of SCN, according to Shires. The critters don’t drown.

“[SCN is] very resilient,” she said. “The wet spring that we’re having right now is probably slowing it down on starting reproduction at the moment, but the summer’s really going to kind of be that determination of how bad SCN is and how many populations it goes through this summer.”

Fields affected by SCN are hard to assess, so when those effects begin to show, the damage is done.

“When foliar symptoms do appear, it kind of looks like a general die-back or decline on the plant, or there’ll be areas of really bad stand-counts or reduced vigor,” said Shires, who is featured on a recent edition of the South Dakota soybean checkoff-sponsored podcast, The Soybean Pod. “That is kind of what we’ll see both in the soil and the foliar symptoms.”

The primary defense growers have against the pest is to plant varieties with the gene referred to as PI 88788, which is bred to resist SCN, but has seen declining efficacy. In many cases, SCN has become resistant to the resistance.

“We know that we have several populations that are becoming partially resistant to that gene,” Shires acknowledged, adding, “We have also done HG typing and found a few populations from a couple of counties that are completely resistant to that gene so that resistance gene will not stop or reduce the production of nematodes.” Put differently, in those cases, the nematodes have free rein, having completely overcome the resistance bred into the PI 88788 soybeans.

HG typing provides an analysis of SCN populations based on their reproduction capabilities among resistant, or formerly resistant soybean varieties. SCN testing, as well as the more involved and pricier HG typing, are paid for through soybean checkoff support from the South Dakota Soybean Research and Promotion Council.

“The funds that we have are so, so helpful,” said Shires, referring to the checkoff support, “and they allow us to employ student workers so that we can get these samples processed and get answers back to the submitters just as quickly as possible.”