The heat and lack of rainfall in July and August resulted in stressful growing conditions for crops in many parts of Iowa this year. And, unfortunately, Iowa's number one soybean pathogen, the soybean cyst nematode (SCN), thrives in hot, dry conditions. "Many Iowa growers saw stunted and/or yellow soybeans develop in mid-to-late July or early August, which is usually when these symptoms occur as a result of SCN damage," says Greg Tylka, an Iowa State University Extension nematologist and expert on these tiny yield robbers.
Research results show in dry soil conditions, SCN juveniles burrow deeper into soybean roots to set up feeding sites in the vascular tissue of the root, where the best possible nutrition is available. It is thought this behavior may be one of the reasons SCN is so much more damaging and seems to reproduce so much more quickly in hot, dry conditions than in cooler, moist growing seasons.
Why is SCN more damaging to soybean yields in hot, dry conditions
Although this summer was hot and dry for much of Iowa, parts of the state received timely rains or even excessive rainfall. Unfortunately, this does not mean SCN will not be a yield-reducing pest in these "wet" areas, says Tylka. Research funded by the soybean checkoff in Iowa and Illinois in the mid-1990s revealed 30% or more yield loss to SCN can occur without any symptoms appearing, and this "hidden" yield loss typically occurs when rainfall is adequate to excessive.
So what does this all mean for Iowa soybean growers? "It means SCN continues to be a threat to profitable soybean production, no matter what the growing conditions are," says Tylka. "It is very easy to sample for SCN in fields, and fall is a perfect time to test fields for the pest."
Good reasons to consider collecting soil samples for SCN in the fall
Tylka says there are are two reasons why you would want to consider collecting soil samples for SCN in the fall:
• SCN is widespread in Iowa and that means fields without SCN continue to be at risk for being infested with the pathogen. So if a field has never been tested for SCN or the soil sample results from a field in past years indicated that SCN was not detected, soil samples should be taken before soybeans are grown again.
It is not possible to predict if every field in Iowa eventually will be invaded by SCN, but it is very likely that SCN spreads into new fields in the state every year. SCN is very easy to keep in check when population densities (numbers) are discovered at low levels, and numbers will be low when SCN first becomes established in a field.
• SCN-resistant soybean varieties typically are grown in fields that are known to have the nematode. The resistant varieties keep SCN numbers from increasing and produce profitable yields despite the presence of the nematode. The resistance is not 100% effective and eventually, SCN populations can build up the ability to reproduce on resistant soybean varieties.
Growers who have managed SCN with resistant soybean varieties for several years should periodically take soil samples following the soybean crops to determine what the current SCN population densities are, and also to gauge if SCN egg numbers are increasing on the resistant soybean varieties.
Is fall soil sampling being done to see if field is infested with SCN?
To assess SCN egg population densities after growing SCN-resistant bean varieties for several years, soil samples should be collected in the fall after soybean harvest. But if fall sampling is being done to determine if a field is infested with SCN, it makes sense to sample the soil in harvested cornfields where soybeans will be grown in 2012. That way, the knowledge of the current status of the field (infested with SCN or not) can be used to plan for next year's soybean crop.
It also is a good idea to collect samples after soybeans are harvested this year if unusual plant growth was observed during the season or if soybean yields were unexpectedly low. If sampling soybean ground in the fall, the soil cores also can be used to assess soil fertility for next year's corn crop.
To sample for SCN in the fall, you should:
• Ideally, samples should be collected using a soil probe.
• Soil cores should be 8 inches deep.
• Collect soil cores from 15 to 20 places in a zig-zag pattern in a sampling area.
• Collect a separate set of soil cores for each 20 acres or so, if possible.
• Combine and mix soil cores, and fill a sample bag with one cup or more of soil.
• Label the outside of each sample bag with a permanent marker.
• If sampling after soybean harvest, collect soil cores from within the root zone of the harvested crop. (No need to do this if sampling in a harvested corn field).
Where do you send your samples? Numerous private soil testing laboratories in Iowa offer SCN analysis of soil samples. Additionally, samples can be sent to the Iowa State University Plant and Insect Diagnostic Clinic, Room 327 Bessey Hall, ISU, Ames, Iowa 50011-1020. The fee for SCN analysis at the ISU clinic is $15 per sample for samples from Iowa. More information about the biology, scouting, and management of SCN can be found at www.soybeancystnematode.info.