Get to the root of SCN problem

We’ve been fighting soybean cyst nematode for over 35 years. SCN is widely considered to be the most damaging pathogen of soybeans in Iowa. Surveys funded by the soybean checkoff and conducted in the mid-1990s and again in the mid-2000s indicate SCN is likely present in 75% or more of Iowa fields.

Get to the root of SCN problem

We’ve been fighting soybean cyst nematode for over 35 years. SCN is widely considered to be the most damaging pathogen of soybeans in Iowa. Surveys funded by the soybean checkoff and conducted in the mid-1990s and again in the mid-2000s indicate SCN is likely present in 75% or more of Iowa fields.

A few more facts: SCN has the potential to increase in numbers very quickly; it can move every way soil moves; it can cause 50% or greater yield loss; and it can survive dormant in soil for a decade or more in absence of a host soybean crop.

We get questions about SCN every year. With the help of plant pathologist Greg Tylka at Iowa State University, let’s walk through some of the most frequently asked.

What are symptoms of SCN? Stunting and yellowing are the most common aboveground symptoms. Unfortunately, that’s also the same symptom of a lot of other soybean issues so it’s completely unreliable for purposes of identifying SCN. You’ll want to dig deeper (pun intended and explained later). Earlier-than-normal maturity of fields is another common and indirect aboveground symptom of SCN.

Belowground we can see root stunting, discoloration and fewer nodules with SCN, but these symptoms are sometimes hard to discern. Unfortunately, noticeable yield losses are often the first indication to a grower that SCN is present in a field if it hasn’t been previously checked.

My fields don’t show any of those symptoms. Do I still need to check for SCN? Yes. Soybean plants often do not show obvious aboveground symptoms of damage when SCN numbers are low or moderate, yet yields can still suffer. The bottom line is because SCN is widely distributed in Iowa, any field that grows soybeans should be checked for SCN.

What if I pick up a new farm and won’t have time to sample, can I just proactively plant SCN-resistant beans? You could, as long as you made sure to choose SCN-resistant varieties that match your other agronomic needs, such as disease resistance. And with the efficacy of some of our resistant genetics fading, start a consistent sampling program as soon as you can even if planting resistant soybeans.

Can I check for SCN now? Yes, and you have a couple of options: root digs or soil sampling. This time of year it’s a great idea to look at both the above- and belowground parts of your soybean plants; so add digging (rather than pulling) roots and looking for SCN females to your scouting. SCN females appear as small, white objects about the size of the period at the end of this sentence.

SCN females first appear on soybean roots about 30 to 35 days after planting, and then can be found on roots throughout the remainder of June, July and into early to mid-August.

After that, SCN females can be difficult to find because they will be appearing on new roots that are growing deep in the soil beyond an easy spade dig. SCN females die after they have fully developed and produced all of their eggs, and they turn brown and form hardened, protective, egg-filled cysts, which are difficult to see on roots.

I’m allergic to digging in summer. Can I soil-test for SCN? Absolutely, and while you could sample soil for SCN about anytime soil isn’t frozen or muddy, fall is a perfect time to do it. It’s a lot like sampling for soil fertility. Many private soil-testing labs can process soil samples for SCN. Samples also can be sent to the ISU Plant and Insect Diagnostic Clinic to be tested for SCN. More information on soil sampling for SCN is available in an ICM news article at extension.iastate.edu/CropNews/2014/1016Tylka.htm.

ISU recommends sampling ahead of every third soybean crop if you know you have SCN and have been managing for it.

What do I do if I find SCN in my fields? If SCN infestations are discovered in fields when nematode population densities are low or moderate, the SCN populations can be kept in check by growing SCN-resistant soybean varieties in rotation with corn. Also, now we have several nematode-protectant seed treatments that can be used when resistant soybeans are grown. If you are unfortunate to have a field test high, you’ll likely have to alter your rotation.

The recommended plan for fields with high levels is to grow several years of a nonhost crop like corn and sample the field every fall.

By planting nonhost crops for a few years, hopefully, you’ll see a decrease in SCN population densities down into the medium or lower category, allowing for soybeans to be put back in the rotation.

Good luck, keep fighting SCN and enjoy those summer root digs.

McGrath is the On-Farm Research and Extension coordinator for the Iowa Soybean Research Center at Iowa State University. Contact him at cmcgrath@iastate.edu.

Soybean cyst nematode problem spurs more questions

Digging soybean roots to check for the presence of soybean cyst nematode females is an easy and effective way to scout for this serious pest. It’s important to look belowground to check for SCN during the growing season. Here are more questions and answers regarding SCN management.

I already plant SCN-resistant beans, so I don’t need to worry, right? I wish that was the case. Unfortunately, much like other pests, SCN is adapting quickly, and the effectiveness of our favorite SCN resistance gene is eroding. For several decades, the PI 88788 source of SCN resistance has produced soybean varieties with greater yields than varieties with SCN resistance genes from other sources, such as Peking. Many soybean varieties with PI 88788 SCN resistance have been developed over the years, and these have allowed farmers to continue to produce soybeans profitably in Iowa for 20-plus years.

But repeatedly using the SCN resistance genes from PI 88788 for two decades or more has resulted in selection of SCN populations with increased reproduction on plants with PI 88788 SCN resistance. Elevated SCN reproduction on resistant soybean varieties leads to SCN numbers “creeping up,” and bean yields heading on a slow and steady decline. It’s estimated currently that more than half of the SCN populations in Iowa have an increased level of reproduction on bean varieties with PI 88788 SCN resistance.

How can we check if SCN numbers are starting to creep up on our resistant soybeans? A “high-tech” approach is to collect soil samples from SCN-infested fields and compare the results of the samples tested at a lab over time, as successive crops of SCN-resistant soybeans are grown. Sampling is best done in fall after harvest, and is the most effective way to track SCN numbers to gauge the relative effectiveness of your resistant beans.

Another way to assess the effectiveness of resistant varieties is to dig roots and look for SCN females on roots during the growing season. Appearance of numerous SCN females on the roots would indicate the resistance isn’t effective. However, ineffective resistance may be because the SCN population has elevated reproduction, the bean variety doesn’t possess all of the necessary SCN resistance genes, or both of these situations combined. If you dig and find SCN females, my recommendation is to start the high-tech sampling protocol to see where SCN levels are trending.

What if it looks like my PI 88788 SCN resistance is not effective? First, don’t give up on soybeans! Soybeans are vital to the economy of Iowa and provide a much-needed second crop in our cropping systems.

Also, farmers battling SCN should continue to use resistant soybean varieties with PI 88788 resistance. Many of these varieties still offer good SCN management and profitable soybean yields. But not all SCN-resistant soybean varieties are equally effective. So some effort is needed to select effective SCN-resistant bean varieties. ISU has the nation’s largest and most comprehensive field evaluation program for SCN-resistant soybean varieties. The program is funded by soybean checkoff funds through a grant from the Iowa Soybean Association. More information about the ISU SCN-resistant Soybean Variety Trial Program is at extension.iastate.edu/CropNews/2014/00117tylka.htm. Seed companies can also be good sources of information about which SCN-resistant varieties offer greatest nematode protection, as well as yield potential.

In addition to selecting good SCN-resistant varieties to grow, farmers battling SCN should seek out resistant varieties with sources of SCN resistance other than PI 88788. Finding these varieties can be “easier said than done”, but try to plant some if possible.

Growing corn is another SCN management strategy to keep SCN in check. SCN numbers may drop as much as 50% in a single growing season with corn, but there is much less of a decline in SCN numbers in second-year corn and the effect becomes even less effective after that. Also, several seed-applied SCN management products are now available for soybeans, and more are in the pipeline.

This article published in the July, 2015 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2015.

Pest Control

Scouting

Hide comments

Comments

  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.
Publish