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Still need to stop the yield-robber

We have come a long way since the early years of managing soybean cyst nematodes in the early to mid-90s, when the SCN Coalition’s motto was: “Take the test, beat the pest.”

At that time, the retail company I was working for was on the front edge of sampling for SCN and working with our seed companies to market the few emerging resistant soybean varieties we had to sell.

First: SCN awareness

Land-grant universities in the Midwest partnered with the Iowa Soybean Association and other state soybean checkoff organizations in the region. ISU also did a tremendous job educating seed companies, agronomists and growers about nematodes, the devastating yield losses they could cause and how to manage them to reduce their impact.

But persuading growers to plant the rather unimpressive-yielding nematode-resistant soybean varieties was a struggle. In fact, in the early years, unless SCN levels were very high, we couldn’t afford the yield hit we took with the early SCN-resistant varieties, so we planted our higher-yielding varieties and watched SCN numbers climb.

Next: SCN varieties

Today, most growers, hopefully, have identified the fields with soybean cyst nematodes and are using the latest SCN-resistant varieties to manage nematodes.

Most of today’s resistant soybean varieties yield as well as or better than the non-SCN-resistant soybeans, in side-by-side yield comparisons, even in the absence of SCN. These varieties also do a pretty good job of decreasing nematode reproduction compared to non-SCN-resistant varieties.

However, not all bean varieties are created equal. I recommend checking out Iowa State University’s SCN-Resistant Soybean Variety Trial Program. The program assesses the agronomic performance of three SCN-resistant variety groups (maturity groups I, II and III) and determines their effects on SCN populations and yields. Results from the ISU SCN-Resistant Soybean Variety Trial Program are posted online at www.isuscntrials.info.

I urge growers to choose varieties based on both yield and their ability to suppress nematode reproduction.

With SCN-resistant bean yields in the ballpark of where we need them to be, a more relevant question today is: Are our SCN-resistant soybean varieties as effective as they used to be?

The most common source of SCN resistance we have is PI 88788, but reports are increasing about it not controlling nematodes as well as in the past. This potential decrease in effectiveness is believed to be due to selection for SCN populations that can reproduce on the PI 88788 resistance over time — not to a decrease in the quality of soybean varieties being grown.

Managing SCN resistance

Checking SCN-resistant varieties in the field may help answer this question, and a good place to start is a midsummer root dig. (Nothing says summer fun like digging plants in July and August!) Because SCN females are easily seen with the naked eye, looking for them on the roots of resistant plants is a crude but useful way to check the effectiveness of resistant soybean varieties in infested fields.

Select the plants to be studied; then start 8 inches from the plant stems and dig about 6 to 8 inches deep. Don’t pull the plants up as this can strip females off the roots. Gently break the soil away from the root system and look for nematode females. They are about the size of a period at the end of a sentence and will appear as small, round, white dots on the roots. Checking plants a few times through July and August is better than doing it once.

If numerous females are observed on roots of resistant varieties, consider doing an HG type test on the nematodes in the field. This is a greenhouse test that assesses nematode reproduction on the different sources of resistance used in breeding SCN-resistant soybean varieties.

Detailed information about type tests for SCN populations can be found in an ISU Integrated Crop Management Newsletter article titled “What’s your type? An HG type test for SCN populations.” To view the article, visit www.extension.iastate.edu/cropnews and search for “HG type test” (it will be listed first in the results).

McGrath is partnership program manager of ISU’s Corn and Soybean Initiative.

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TINY THIEVES: Check fields for SCN by looking for white SCN females on soybean roots at this time of year. Guidelines were discussed in ISU’s June 2011 ICM Newsletter article “Females now apparent on soybean roots.”

This article published in the August, 2011 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.