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 .
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.
This article published in the July, 2015 edition of WALLACES FARMER.
All rights reserved. Copyright Farm Progress Cos. 2015.