Is that Really SDS in a Dry Year?

By Daren Mueller

I have seen yellow patches in Iowa soybean fields this past week. Plants within these patches have foliar symptoms that appear to be caused by sudden death syndrome (Figure 1; SDS). When talking to the farmers, they are a bit surprised to see these symptoms as seeds in these fields were treated with an “SDS seed treatment” and dry conditions have been present throughout much of the state in 2021.

A few thoughts/observations…

• I have observed the most SDS on soybeans with new herbicide traits. From past observations, when new traits come to market, they often are introduced in soybean cultivars focused more on yield/standability (not disease resistance).
• Are the SDS seed treatments working? We obviously want to see reductions in SDS, but most of the fields I have been in do not have check strips, so there is no way to know how much disease would have been there without the seed treatment. In past years, we sometimes still see significant differences in yield between treated and non-treated seed even if there were no obvious differences in foliar symptoms. This may be due to delaying foliar symptom development or reducing the root rot phase of the disease. We have some of these same seed treatments in our small-plot trials, so I am curious to see if this same trend will play out at harvest this year.
• Some of the yellow patches that hardly look like SDS are indeed still SDS. The lack of rain here in parts of Iowa has resulted in some very non-traditional disease symptoms. I think this is the impact of the root rot phase. Roots are rotted in these patches with stunted and yellow plants, but the yellowing is not from the toxin, but perhaps from the root rot phase of SDS.
• If you have SDS this year, take note of it because it has not been an ideal year for SDS. However, split stems because I am also seeing some possible brown stem rot in fields (Figure 2; we are doing isolations this week to confirm). Before you make conclusions about how well your cultivar and seed treatments are holding up, be sure you have the correct disease!

Figure 1. Yellowing and death of leaf tissue symptomatic of sudden death syndrome in soybean.

Figure 2. Split soybean stems to help tell the difference between SDS and diseases that have similar foliar symptoms such as brown stem rot.