Palle Pedersen, Soybean Extension Agronomist
Originally published in the Iowa Soybean Review, November 2005
Looking back at the 2005 growing season, we start to question why we were not always able to achieve the high soybean yield we know we can get. Although some parts of Iowa had extremely good crops, eastern Iowa faced one of the worst droughts in many years. In addition to the drought, they have been fighting spider mites, an uncommon guest in Iowa except during drought years.
I am doing a lot of research in eastern Iowa, and it has not been easy for me either. However, research is research. Why should it be easier for me than it is for you?
On August 11, I drove through the Eastern Iowa area to meet up with my research collaborator. All the fields in his neighborhood were dying fast. Both he and his neighbor were blaming the drought and spider mites for the worst fields they had ever seen.
After digging up a few plants with 40 to 60 cysts per plant, though, they realized that the early death of their soybean this year was not entirely because of the drought and spider mites.
The soybean cyst nematode (SCN) is the single most damaging pest affecting soybean in the United States, where annual yield losses due to SCN have been estimated at about $1.5 billion. It was estimated that the state of Iowa alone lost 50 million bushels to SCN in 2004, and that was in a wet year, when yield loss from SCN often is not as high as dry years. I expect that losses will be much greater this year.
The microscopic, soil-borne pest is present in more than 75% of the fields in Iowa and can reduce soybean yields by 50% or more. Unfortunately, there are often no obvious aboveground symptoms of damage caused by SCN. Only under drought conditions will aboveground symptoms typically appear. Consequently, many infestations go undiscovered for numerous years.
Seed company personnel informed me that less than 40% of the soybean seed sold in Iowa this year had SCN resistance, despite the fact that more than 80% of Iowa's fields are infested. And there are no additional costs for selecting SCN resistant varieties.
The problem is that farmers can still remember the early 1990's, where there was a yield drag between SCN-resistant and SCN-susceptible varieties in fields with no or low population densities of SCN. That is not the case anymore. Today, the best SCN-resistant soybean varieties are yielding equal to the most SCN-susceptible soybean varieties in fields that are not infested with nematodes. In addition, resistant soybean varieties limit nematode development, resulting in no increase, or at times a decrease in population densities of the nematode.
The difficult part when selecting SCN-resistant varieties is this: SCN-resistant varieties that yield comparably do not necessarily control the nematode equally. Selecting SCN-resistant varieties based solely on yield data is shortsighted and risky, because some relatively high-yielding, SCN resistant soybean varieties can still build up the nematode population densities. Yes, the long-term consequences may be much larger than you really think.
The best way today to select SCN-resistant varieties is using information from the ISU SCN variety trial conducted by ISU Extension Nematologist Dr. Greg Tylka.
An ideal time to sample fields for soybean cyst nematode is in the fall, after harvest and before the soil freezes. The only time soil samples should NOT be collected is when soil conditions are very wet. Keep the samples away from sunlight in a cool area until shipped to either a commercial laboratory or to the Plant Disease Clinic, Iowa State University, 323 Bessey Hall, Ames, IA 50011.