The availability of high-yielding varieties resistant to soybean cyst nematode (SCN) may be too much of a good thing.

Just as overuse of herbicides with the same mode of action can bring about resistant weeds, the overuse of SCN-resistant varieties may set your fields up for a race shift.

Plant pathologists emphasize there has so far been no evidence of population changes or breaks in resistance. But nematode experts warn that it will be better to prevent the problem by managing SCN with crop rotation and alternating varieties with different sources of resistance.

Iowa State University plant pathologists explain that less than 1% of SCN juveniles will reproduce on the current resistant varieties. If you keep growing SCN varieties with the same source of resistance, eventually the SCN juveniles will be able to reproduce as well on the resistant varieties as on susceptible ones. However, throwing a susceptible variety into the mix can disrupt the growth of the resistance-feeding juveniles.

There are seven sources of resistance being used in breeding programs, says Terry Niblack, a University of Missouri plant pathologist. Four of those sources are available in today's varieties.

They are PI88788, Peking, PI437654 (also called Hartwig) and PI209332. The PI88788 and Peking are the most common sources of resistance. In Iowa, for example, 98% of the SCN varieties are from PI88788. Three varieties have Peking resistance. The Hartwig resistance is available only in Hartwig, and Faribault has PI209332 resistance.

Most of the information about the sources of resistance is readily available from private and public breeders.

The first step to manage SCN and resistance is to verify, with nematode counts from soil samples, that you have the problem - and how badly. Too many farmers are choosing to grow SCN-resistant varieties without any confirmation of SCN in their fields or on root inspections, rather than soil assays, say pathologists and agronomists. You can find female cyst nematodes on soybean roots in July and August, but nematologists want soil samples for positive identification.

Depending on the severity of SCN in your fields, you can tailor your crop rotation using a four- or six-year system.

That means starting with a non-host crop such as corn, following the second year with an SCN-resistant bean variety. The third year plant corn again, and follow the fourth year with a variety with a different source of resistance to SCN. Rotate to corn in the fifth year, then plant a susceptible variety in the sixth year.

The best option for SCN-resistance rotation is to use varieties from different resistance sources, says Greg Tylka, an Iowa State extension nematologist.

The second-best choice: Alternate varieties that have the same source of resistance. The worst-case scenario Tylka cites is growing the same variety in consecutive years.