SW MN IPM Stuff 2021 Issue 1

Volume 24 Issue 1 | February 9, 2021

This newsletter and the advice herein are free. You usually get what you pay for.


Crop weather

Rainfall, air and soil temperatures, degree-days, soil moistures, and other current and historical weather data for the University of Minnesota Southwest Research and Outreach Center (SWROC), a little spot about two miles west of Lamberton, MN, can be found at SWROC Weather.  The ongoing cold snap may have an impact on overwintering insects such as western corn rootworm, particularly in fields with little snow cover. We will have to wait and see what happens to soil temperatures.

SCN resistance sources

I don't work much with soybean cyst nematode (SCN) anymore. Recently, however, curiosity and a request from a Minnesota Soybean member “encouraged” me to find out if many soybean varieties containing SCN resistance sources other than PI 88788 were available for MN and how much that had changed from a year ago. This topic was also briefly addressed during the recent “Soybean Pre-plant Pest Management Decisions” SWROC Advancing Ag webinar.

To satisfy my curiosity, I looked through the seed company information on the web, sent a few emails, and made a few calls. There were only three companies that made a web-search on SCN resistance sources a less than painful experience. On the other hand, searching on herbicide trait(s) was a piece of cake. I suppose that makes some sense as SCN are less likely to kill a crop than the wrong herbicide applied to the right variety or vice versa! The SCN resistant varieties without PI88788 I found are listed in Table 1. Because herbicide tolerance is apparently a greater driver for variety selection than SCN resistance, it was included to help ward off any dramatic visual and economic issues in 2021 soybean fields.

In summary:

  • There are still relatively few varieties with SCN resistance other than PI 88788.
  • None of these were maturities adapted to Northern MN. 
  • Some of the varieties listed may be the same variety with different labels. 
  • Few of these varieties have been screened for level of resistance by the UMN. See UMN 2020 field crop trials for those that have.
  • There are two varieties available with the PI 89772 resistance source.

As SCN populations that can reproduce (are virulent) on resistant varieties become more prevalent, agriculturalists need to get deeper in the weeds on SCN management. The SCN populations in some fields can now reproduce on soybean varieties with one or more resistance sources. These resistance sources are derived from soybean plant introduction (PI) lines with unique resistance gene combinations. Currently, most SCN resistant varieties contain resistance from PI 88788, a few are Peking (PI 548042) and fewer still are PI 89772. In my quick search, I could not find any currently sold varieties with PI 437654 (Hartwig/CystX) resistance.

These reproducing populations can be identified as either races or as HG types. Only four soybean varieties were used when the SCN race type system was developed (Riggs, et al., 1988) and as a result, this system has some limitations. The HG type system (Niblack, et al., 2002) is limited only by sources of resistance available to test and is better at identifying populations resistant to multiple resistance sources. The imperfect relationship between SCN race and HG type can be found at: Converting SCN races to HG types (The SCN Coalition).

Seed companies most often use SCN races in their variety descriptions. For example, PI 88788 varieties might be listed as resistant to SCN Race 3 and Race 14. Peking might be listed as resistant to Race 1 and Race 3. Any SCN that would reproduce on the Syngenta varieties with the PI 89772 resistance would be HG type 6. In the older race system that is used to describe SCN resistance, there is no corresponding race for HG 6. Based on surveys of SCN in MN, HG Type 6 was highly correlated with HG Type 1, but not HG 2.  This can be translated to the varieties with PI 78772 resistance are unlikely effective for SCN populations that can overcome Peking varieties, but may be resistant to the SCN populations that are able to overcome PI 88788 varieties (Senyu Chen, pers. comm.).

Except for Northwest Minnesota, HG type 2.5.7 (could be race 1 or race 5) is relatively common in soybean growing areas of the state. Soybean fields with these SCN populations may see an unacceptable population increase and yield loss if PI 88788 resistant soybeans are planted. If you suspect resistance issues and want to know what flavor of nematodes you may be dealing with, the University of Minnesota Nematology Lab can do an HG test (see SCN soil sampling). Contact the lab first as HG tests require a much larger volume of soil than a typical test for SCN egg population density.

Remember, overuse of any resistance source can select for resistant SCN populations (See SCN variety rotation).

Table listing commercial SCN varieties adapted to Minnesota. Table includes company name, soybean variety and maturity group, SCN source, and herbicide trait
Table 1. Commercial SCN resistant varieties adapted to MN with a resistance source other than PI 88788. This may be an incomplete list.

Products mentioned are for illustrative purposes only. Their inclusion does not mean endorsement and their absence does not imply disapproval.

Happy trails,

Bruce Potter