SW MN IPM STUFF 2018 Issue 1

Volume 21 number 1

03/15/2018

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

A printer-friendly pdf version of this newsletter is available to download here: SW MN IPM Stuff 2018-01

Crop weather

It is still winter-like out there but hopefully not for long. Contrary to some of the rumors spread by farmers returning from the south, I have not been fortunate enough to be able to spend the winter twiddling my thumbs and otherwise taking it easy.

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.

Black Cutworm

We will be sending the traps to cooperators soon. We are still looking for cooperators to run a trap in Houston, Winona, Kandiyohi, Big Stone, and Traverse counties. It’s a pretty easy gig, so if you or someone you know would like to run a trap, contact Travis Vollmer at tvollmer@umn.edu. Might make a good 4H or FFA project for someone. 

SCN and resistance varieties…It’s just a little clearer than mud.

I received a few questions on how the Hg test relates to the older soybean cyst nematode (SCN) race descriptions found in most current seed company literature. I’ll try to help but no longer much work on SCN. Any real nematologists out there should feel free to join in the discussion.

Most of the SCN resistant varieties currently planted are derived from PI88788. A few varieties are from PI548402 (Peking) and a very few from PI437654 (Hartwig/CystX®). For the more curious readers, PI stands for plant introduction.

As SCN populations capable of attacking resistant varieties were discovered, a system to describe which resistance sources that these SCN were virulent on (capable of surviving, damaging and reproducing on). Initially these populations were described with a system of SCN races. The races (Table 1) were based on which of the resistant sources (Pickett, Peking, PI88788 and PI90763) would allow SCN reproduction of 10% or more than the reproduction on a susceptible variety. The varieties were planted into a known concentration of SCN cysts (or eggs), and then counting the SCN produced the roots of plants have grown in the greenhouse. Race 3 SCN can only reproduce well on a susceptible variety, Race 1 on the susceptible and PI 88788 differential. Some races can reproduce on more than one resistance source. For example, race 2 can reproduce on the susceptible, Peking and PI88788.

Now here’s where things get complicated: Pickett, one of the 3 SCN resistant differential varieties used in this older 16 race system (Riggs, et al. 1988) was derived from a cross with a susceptible (Lee) and Peking. Therefore, the resistance in Pickett would contain some or all of the same genes as Peking and some races should not actually exist. 

Additionally, other resistance sources, PI437654 (found in Hartwig and CystX®) for example, are not accounted for.

A better method to determine which SCN resistant sources will control a particular SCN population was needed. Niblack et al. (2002) developed Hg (short for Heteroda glycines) types, a new system for describing how SCN interacts with a susceptible variety and differential varieties with seven (7) resistance sources (Table 1). SCN virulence (ability to infest) on these seven resistance sources is still based on 10% or more reproduction compared with a susceptible. Hg0 is susceptible to all resistance sources, Hg1 is virulent on Peking, Hg2 on PI88788, and Hg3 on PI 90763. An SCN population capable of reproducing on more than one differential variety is described by each individual resistance source; an Hg 2,5,7 SCN population is virulent on PI88788, PI209332, and PI548316.

Table 1. Differential SCN resistant sources, Hg types, and races used to describe SCN virulence on SCN resistant varieties.

The Hg type system does a better of describing which varieties will work against a particular population of SCN and the old system of 16 races has fallen out of favor.

Remember, these Hg types are a description of a population of SCN, which often has considerable genetic diversity! Even though a test does not show a particular Hg type, Hg1 for example, these biotypes can be present a low frequency and will be selected for as you use that resistance source. Managing resistant SCN populations is a bit like dancing with the devil. You start getting in trouble with the first dance and every dance it gets increases your risk.

Hg types can be conducted on field collected Hg samples. The U of MN Nematology Lab will run these tests but since the tests are conducted on growing soybean plants, tests are not run year round and results take a while. For more information, visit SROC Nematode Soil Sampling.

Table 2. A comparison of Hg types and races used to describe SCN virulence on resistant sources. Non-bold races are unlikely to exist.

So to most simply answer the original question, the two systems can be partially related, but on a very limited basis for susceptible SCN populations and SCN virulent on three of the several resistance sources, PI88788, PI548402 (Peking), PI90763 (Table 2). 

To relate this SCN discussion to entomology, consider that some biotypes of soybean aphid are known to be virulent on one or more Rag (resistant to Aphis glycines) genes in soybean resistant varieties. As additional resistance genes are incorporated into soybeans and virulent soybean aphid populations are selected, like SCN, describing these populations will become more difficult.You can get an idea of the virulence of SCN in a field on an SCN resistant variety (more than one if you plant strips of resistant varieties). Compare the egg counts from soil samples at planting and at the end of the season. Take your soil samples in the same areas at the beginning and end of season, keep the area(s) sampled small, and take more than one sample for each comparison.

Happy Trails,

Bruce Potter