Now is the time of year when farmers should be sampling fields and testing the soil for the presence of soybean cyst nematode. SCN is a tiny, yield-robbing, almost invisible worm that attacks soybean roots and steals yield.
"After harvest is the best time to do this sampling and testing," says Greg Tylka, Iowa State University Extension nematologist. "SCN continues to be a threat to profitable soybean production, no matter what the growing conditions are, whether it is a wet year or dry year."
There are also nematodes that feed on roots of corn plants, but now is not the time to sample fields for corn nematodes. That's best done in summer, he says. However, fall is the perfect time to pull soil samples and have them tested for soybean cyst nematode. With the information you get back from the lab based on the soil samples pulled from your fields, you can plan ahead and make wise choices regarding soybean variety selection to control this pest. The test results will show what the SCN population is in each field.
What's best bet for controlling soybean cyst nematode in your fields?
Planting soybean varieties that have resistance to SCN and managing them correctly is one of the most effective things you can do to control this pest. Are Iowa farmers making progress in the fight against these tiny troublemakers? "Yes," says Tylka. "Sometimes when you're out in the field looking at your bean yields, and they aren't increasing like your corn yields are, it doesn't seem like we're making progress. But we are making progress controlling SCN."
Recently, Tylka summarized research that has been conducted at ISU on SCN over the past 20 years and it shows tremendous gains have been made. Iowa soybean farmers have funded most of that research through the soybean checkoff. "Thanks to these studies, we now know so much more about how this pest spreads, what factors it prefers for high reproduction, how SCN affects soybean plants, what diseases it interacts with and other things we didn't previously know about SCN," says Tylka.
Today, there are hundreds and hundreds of SCN-resistant soybean varieties on the market, available for growers to plant, which have a suitable maturity rating for growing in Iowa. Tylka recommends farmers alternate their soybean varieties, planting an SCN-resistant variety one year, then rotate to corn, then the next year you plant soybeans again in that field, you should use a soybean variety that has a different source of SCN resistance.
What different types of SCN resistance are available in soybean varieties?
There are different types of SCN resistance in soybean varieties. "We call those 'sources' of resistance," says Tylka. "It used to be that almost every soybean variety on the market that had SCN resistance in it was getting that resistance from one primary source, called PI 88788. But now there are other sources of SCN resistance available which are being used by soybean breeders to provide the resistance in soybeans. However, the PI 88788 resistance is still being widely used in many soybean varieties."
Every year at the end of harvest Tylka puts together a list of SCN-resistant soybean varieties. He prints it and distributes it through ISU Extension to help farmers make informed choices when deciding which SCN-resistant soybean varieties to plant. This year he has 809 SCN-resistant soybean varieties listed in the publication, and the varieties go across maturity groups I, II and III. Those are the three maturity groups that are planted in Iowa. He says 791 of those 809 varieties have the PI 88788 type of genetic resistance.
The widely-used PI 88788 resistance in soybean varieties is not failing
"So here we are 20 years into the battle against SCN in Iowa and we still have one main type of resistance to SCN that we are using in these soybean varieties," says Tylka. "However, by telling people that fact, I don't want to turn growers off from using that resistance. The PI 88788 resistance still works great. True, the soybean cyst nematode is starting to build up somewhat on that type of resistance. However, we at ISU conduct soybean variety trials each year and we still see 65 to 70 bushel per acre yields being produced by good, SCN resistant soybean varieties that contain the PI 88788 resistance."
Tylka continues, "The point is, this PI 88788 resistance is not failing. It still works well. But if a grower could alternate his planting of soybeans with another type of resistance, like the Peking type of resistance that is in some soybean varieties, that would be a very wise move." Thus, you'd plant a PI 88788 SCN resistant variety of beans one year and the next time that field is in soybeans you would plant a variety that has the Peking source of resistance.
Will corn-soybean crop rotation help lower SCN numbers in a field?
What about crop rotation? Will planting two or more years of corn between the soybean crop in a field do more to lower SCN populations that have built up? Or will one year of corn between bean crops suffice? Iowa has certainly seen more corn-on-corn being grown in the state in recent years, as the ethanol industry has increased demand for corn. Still, the most popular crop rotation is one year of corn followed by one year of soybeans.
"You definitely want to rotate your soybeans," says Tylka. "Without a doubt you should avoid planting soybeans two years in a row in the same field. One year of corn between the soybean crops in a field will drop SCN numbers in the field by 40% to 50%, our studies show. This one-year rotation of corn between soybeans also reduces the activity of other soybean pathogens or diseases in the field."
The best bet is to not grow continuous soybeans, continuous corn or continuous anything—from a disease and pest control standpoint. "Growing continuous crops in the same field is the worst thing you could do in terms of encouraging diseases and pests," says Tylka. "We like to see one year of corn being grown between soybean crops. One year of corn is enough between soybean crops to help keep soybean cyst nematode numbers in check."
List of 809 different soybean varieties resistant to SCN is available
Tylka says you would never be able to totally eliminate SCN in a field by growing continuous corn there, because once the SCN eggs start hatching for a year or two and find no soybean roots to chew on, the pest goes dormant and it can survive for 10 to 15 years in the soil. That means you're not going to get rid of SCN by growing continuous corn in a field, but keep in mind that one year of corn alternated with one year of soybeans is an effective SCN management rotation.
So, get your soil sampled now, and have it tested for SCN, he advises. What you learn from the SCN soil test results that come back from the lab will help you make plans this winter and aid you in choosing which soybean varieties to plant in spring 2012.
Sampling and testing for SCN this fall gives you time to plan for 2012
What if you don't get the soil sampled for SCN this fall? "You can sample soil in the spring too, for SCN testing," says Tylka. "However, fall is a great opportunity to get soil sampled and have it tested. That gives you more time to think about your SCN control options and plans for next year. What do you want to do for 2012?—in terms of varieties of soybeans to plant, rotation, seed treatment, etc."
Tylka's list of 809 SCN-resistant soybean varieties suitable for planting in Iowa maturity zones is available on the Iowa Soybean Association website (click the production research tab) . The ISU Extension publication listing the 809 SCN resistant soybean varieties is also available from ISU Extension's Online store for new ISU publications. It is ISU Extension publication PM 1649, "Soybean cyst nematode-resistant soybean varieties for Iowa."
Summertime is the best time to sample soil for corn nematodes
What recommendations does Tylka have for controlling corn nematodes? While summer is the best time for sampling for corn nematode, it can be frustrating. The crop is already in the ground and you've paid for the inputs, and there's not much you can do by then if you find out you have a nematode problem in corn.
"So for corn, we're almost always fighting the nematode battle six months into the future," says Tylka. "Summer is when you want to scout for nematodes on corn. Get good soil samples. Have them tested. If you discover you have a problem with nematodes in cornfields, then you can develop a plan of action for the next time you are going to grow corn that field."
There are now two options available to control nematodes in corn
What should you include in your plan of action to combat corn nematodes? Up until a few years ago there were just one or two insecticide-nematicide products on the market you could use. There is still one available, called Counter, an insecticide-nematicide. It can be applied in-furrow at corn planting.
"A few years ago we opened a brand new book in nematode control," says Tylka. "That is, seed treatments came on the market which provide early season protection against nematode feeding. The first one that became available was from Syngenta and it is a product called Avicta. This seed treatment was widely available for Iowa growers in 2010. Then during the 2011 growing season, a second entry into the marketplace, called Votivo, was introduced. Those two seed treatment options, as well as the soil-applied Counter insecticide-nematicide product, help combat nematodes that can feed on corn plants."