Combating SCN: Weakening of PI 88788 Resistance

Overreliance on PI 88788 as the major source of genetic resistance has weakened its effectiveness as soybean cyst nematode (SCN) populations continue to evolve.

SCN is the the single largest biotic stress to soybeans in Illinois. The North Central Soybean Research Program (NCSRP) estimates disease losses in Illinois and suggests that soybean producers in the state lose 18 to 20 million bushels annually to this pest.

Growers know the strategies to combat SCN are primarily rotation and genetic resistance. Today there are a few seed treatments, such as VOLTiVO (+ IleVO) and Clarvia, that help. But unfortunately the genetic resistance we’ve counted on is not as effective as it once was.

Currently there are three sources for genetic resistance to SCN; PI 88788, PI 548402 (Peking), and PI 437654 (Hartwig and CystX). The PI 88788 source is used in over 90 percent of existing SCN-resistant varieties being marketed. Only a few varieties use the PI 548402 and PI 437654 sources and they are not readily available.  PI 88788 was able to reduce nematode reproduction by around 80 to 90 percent. Unfortunately its effectiveness has gone down.

I reached out to Jason Bond, plant pathologist at Southern Illinois University about the status of

PI 88788. I asked what happened to PI 88788’s effectiveness since it used to reduce SCN reproduction by 80 to 90 percent and particularly on race 3, which is the most common race in Illinois.

Bond said that effectiveness depends on four things.

1. The amount/density of SCN in the soil (attacking the roots on a particular plant)
2. How aggressive the local population is.
3. The HG types of SCN
4. The level of resistance in resistant varieties varies (complement of resistance genes varies)

Bond explains that HG type is not a static measurement: “For instance, HG Type 2 (known as Race 2 or 5) designation means that in a virulence test in the greenhouse, PI 88788 would support at least 10 percent of the reproduction compared to a susceptible variety.  Most fields (>70 percent) in Illinois now have HG Type 2, but if you were to run them in a virulence test, they would vary in how many individuals in the field populations could reproduce on PI 88788.

“I estimate you would see the populations reproducing on PI 88788 at a range of 10 to 40 percent of the reproduction that you would see on a susceptible variety. Bottom line is two farmers may both have SCN HG Type 2 populations in their fields, but one could be a lot more aggressive than the other population.”

So resistance to PI 88788 varies and its effectiveness overall has declined. Bond added: “For fields with a Type 2 population I would say if the farmer is rotating with a non-host like corn every other year, and using variety with PI 88788 resistance genes being expressed, and if his SCN egg density is less than 2,000 eggs/100 cc at planting, the resistance will probably 70 to 85 percent effective. If you get into fields with populations higher than 3,000 eggs/100 cc soil, then the drop-off of effectiveness could be much more severe.”

To learn more on HG typing read Soy Envoy Stephanie Porter’s blog.

Soybean agronomist Daniel Davidson, Ph.D. posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or ring him at 402-649-5919.