Soybean cyst nematode: how and when to sample

What the decision depends on

SCN costs North American soybean growers more than $1 billion a year, and most infested fields show no symptoms. In a moist season an infested field can look fine and still lose yield; above-ground signs, when they appear, are patches of shorter, yellower plants, often stretched in the direction of tillage. The only unique sign is the adult female, a lemon-shaped, cream to tan body on the roots, far smaller than a nitrogen nodule, and not seeing females does not mean the field is clean. Soil testing is the reliable method.

The number drives three decisions: whether to grow soybeans in that field next year, which source of SCN resistance to plant, and whether a nematode-protectant seed treatment is worth paying for. Later samples tell you whether the plan is working, which is why the first sample is a baseline and the later ones are the useful ones.

Step-by-step: collecting the sample

1. Time it. Fall after harvest is best. Egg numbers are highest then, and a fall count from a soybean field explains this year's yield while a fall count from a corn field predicts next year's. Spring sampling is acceptable but tends to read lower after winter die-off and early hatch, so it can understate the threat. Consistency between years matters more than the exact date.

2. Divide the field. Keep each composite sample to 10 to 20 acres (4 to 8 ha), and never more than 20 to 25 acres. If you already use management zones or 5- to 10-acre grids for fertility, use the same boundaries so the SCN result lines up with your yield and soil data. One large sample from a big field is still better than none.

3. Take the cores. Use a soil probe, not a spade. Collect 15 to 20 cores per area (Nebraska says 15 to 25) from the surface to 6 to 8 inches, in a zigzag at fixed intervals regardless of how the crop looked. Pull each core from within a few inches of the old stem line, in the root zone, where cysts and eggs concentrate. Avoid saturated or frozen soil.

4. Target the hot spots. Add a separate sample from field entrances and equipment paths, low spots and areas that flood, edges next to infested fields, high-pH or sandy areas, and anywhere soybeans were stunted or yellow or yield was unexplained. Water and machinery move cysts, so these are where SCN arrives first.

5. Mix, bag and label. Mix all cores from one area in a clean bucket and fill a bag with about two cups (roughly 500 mL) of soil, labelled with field, area and date. If you are splitting a fertility sample, mark the SCN portion clearly. Samples do not need refrigeration; keep them out of the sun and ship promptly.

6. Submit it. University diagnostic clinics and private soil labs run SCN egg counts for a fee, and some states and provinces run free or checkoff-subsidised programs through The SCN Coalition and their soybean boards. Request results as eggs per 100 cc (US) or per 100 g (Ontario), not cyst counts alone.

Reading the report

State and provincial labs use slightly different ranges, so read yours against the guide that comes with the report. Three published scales:

Ontario reports eggs per 100 g of soil: under 1,000 is low risk (0 to 20 percent potential loss on a susceptible variety), 1,000 to 10,000 is moderate to high (20 to 50 percent, and PI 88788 varieties will likely be affected), and over 10,000 calls for two years of non-host crops before resampling.

Two cautions. A zero does not prove the field is clean; SCN is patchy and a low population can be missed, so keep sampling. And populations move fast: NDSU recorded a rise from about 1,000 to 30,000 eggs in one season in an unmanaged field. Treat any positive as the start of active management.

HG type tests

An egg count says how many; an HG type test says which. The lab grows the population on a set of indicator soybean lines in the greenhouse and reports a female index, the number of females on each line as a percentage of those on a susceptible check. Below 10 percent, the resistance in that line still works against your population; above it, the population can reproduce on it. An HG type 2 population reproduces on PI 88788, the resistance source in 95 percent or more of SCN-resistant varieties sold in Iowa since 2006 and over 90 percent of entries in Minnesota trials.

That is the problem. In Iowa State's 2022 sampling, three of eight research fields had female indices of 75 to 90 percent on PI 88788, three had 50 to 68 percent, and no Iowa population had exceeded 70 percent before. Peking-source resistance has not shown the same increase. If your egg count is high, or is not falling under resistant varieties, an HG type test tells you whether PI 88788 is still doing anything in that field. Not every lab offers it; ask.

Management: what to do with the number

Rotate to non-hosts. Corn, wheat and other small grains, and sunflower are non-hosts. Rotation reduces egg numbers but does not eliminate them; plan at least one non-host year between soybean crops, more at high counts. Control winter annual hosts such as purple deadnettle, and avoid cover crops that host SCN, including several clovers, cowpea and common and hairy vetch.

Rotate the resistance source. Do not plant the same source in consecutive soybean crops. Alternate PI 88788 varieties with Peking varieties, even for one season, and within PI 88788 pick varieties with the strongest documented SCN control. One year of a susceptible variety can undo five years of progress. Check the brown stem rot rating when moving to Peking, because some Peking varieties are BSR-susceptible.

Consider a nematode-protectant seed treatment where counts are high and the economics justify it, and evaluate it with a check strip; responses vary.

Resample the same areas every soybean cycle, or annually in high-count fields. Egg counts should hold or fall under the plan. If they climb under PI 88788, that is your HG type 2 signal.

US and Canada differences

The biology is the same; the reporting units and the registrations are not. Ontario labs report eggs per 100 g rather than per 100 cc, with the OMAFRA thresholds above, and Ontario's Agronomy Guide (Publication 811) carries the provincial sampling protocol. SCN is confirmed in Ontario, Quebec and Manitoba. Seed treatments and nematicides must carry a Health Canada PMRA registration and PCP number; a US label does not apply in Canada. The SCN Coalition, led by university nematologists in 28 states and Ontario, hosts sampling guidance, a list of testing labs, resistant-variety lists and the Profit Checker yield-loss estimator, built on more than 35,000 Iowa State variety-trial plots.