Nematode Rotation & Starve Them Below Threshold

You choose the nematode species, enter your starting population (from a soil assay) and build a rotation sequence of crops. Each crop multiplies the population by its reproduction factor Rf for that species — greater than 1 on a good host (build-up), below 1 on a non-host, resistant cultivar or fallow (decline) — and an over-winter decay is applied each season. The tool projects the population season by season and flags when it falls below the species damage threshold.

Rf is the final population divided by the initial population (Pf ÷ Pi) over one season of a crop. An Rf above 1 means the nematode multiplied on that crop (a good host); an Rf below 1 means it declined (a poor host, non-host, resistant cultivar or fallow). Multiplying the Rf values across a rotation projects how the population changes — the core of nematode rotation planning.

It depends on how high you start above the damage threshold and how low the Rf of your chosen crops is. A strong non-host or resistant crop with an Rf around 0.3–0.5 roughly halves the population each season, so a population two to four times the threshold often clears in two to three seasons. The tool reports the exact season at which your sequence first drops below the threshold.

The damage (action) threshold is the population below which yield loss is minimal. In this dataset it is about 100 J2 per 100 cm³ soil for root-knot, 2,000 eggs per 100 cm³ for soybean cyst, 200 per 100 cm³ for root-lesion, and only 50 J2 per 100 cm³ for Columbia root-knot, which damages potato tubers at low counts. Rotation aims to push the population below this line before you plant a susceptible crop.

Strong non-hosts and resistant cultivars with the lowest Rf. For root-knot, marigold (Tagetes), resistant cowpea, sorghum-sudangrass and resistant tomato are powerful; for soybean cyst, non-host corn and resistant soybean cut egg counts; clean weed-free fallow declines all species. The tool surfaces the single best non-host (lowest Rf) for your chosen species so you know the most effective rotation crop.

The population multiplies every season — an Rf of 4–8 on a good host means a fast build-up to damaging and then yield-crushing levels. The tool shows this 'continuous susceptible' trajectory alongside your rotation as a warning, making the cost of not rotating explicit: the same field that clears in a few seasons under non-hosts can become unfarmable for the susceptible crop if cropped to it continuously.

Almost — a resistant cultivar carries a very low Rf (often 0.3–0.6) because the nematode cannot reproduce well on it, so it drives decline like a non-host while still giving you a marketable crop. The caution is resistance breakdown: rotate resistance sources (for soybean cyst, alternate PI 88788 with other sources) to avoid selecting virulent populations, which the species notes in the dataset highlight.

Free-living juvenile stages die off naturally between crops even without a host, through predation, starvation and weathering. The model applies a species-specific over-winter decay factor each season on top of the crop's Rf, so a non-host year combines the crop's suppression with this natural decline. Cyst nematodes, protected inside cysts, decay more slowly than root-knot juveniles, which the per-species factor reflects.

Clean, weed-free fallow declines all the species here (Rf around 0.45–0.55 per season) because there is no host to reproduce on, but it forgoes a crop and only works if you control weed hosts, which many nematodes also feed on. A resistant or non-host cash or cover crop usually beats bare fallow because it suppresses the nematode AND returns value or soil benefit — the tool lets you compare both in a sequence.

Rotation with non-hosts and resistant cultivars is the durable, lower-cost foundation; nematicides give a fast single-season knockdown but are expensive, regulated and do not change host status. If your population is far above threshold and you must plant a susceptible crop next season, a nematicide may bridge one season, but the tool's multi-season projection shows whether a rotation can get you below threshold without it — usually the better long-term plan.

They are representative mid-points of published host-status and population-dynamics trials, so they give a sound planning projection rather than an exact field outcome. Real decline varies with temperature, moisture, weed hosts and the specific resistance source, and host status can differ by local population (HG-types for soybean cyst). Confirm host status for your region and verify with a follow-up soil assay before planting the susceptible crop.

To get a meaningful starting population, yes — a soil assay reporting eggs or juveniles per 100 cm³ gives the Pi the projection starts from. Without it you can still explore how different rotations compare in relative terms, but the seasons-to-clear answer is only as good as the starting count. Re-assaying after a rotation cycle confirms whether the field has actually dropped below the damage threshold.