MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.    

As if soybean growers didn’t have enough to worry about with soybean cyst nematodes, aphids, rust and the lousy economy, along comes another potential issue. To qualify right up front, this problem is far less severe than the economy because it addresses another nematode and we all know nematodes rank just under the economy. However, laboratory evidence suggests lesion nematodes may become important in soybean production as resistance breakers. Soybean varieties with soybean cyst nematodes-resistance may be more susceptible to soybean cyst nematodes on sites where lesion nematodes are abundant.

Lesion nematodes are probably the most common type of plant-parasitic nematode found in Michigan, The most common species in the state, Pratylenchus penetrans, is a parasite of virtually all cultivated plant species. Essentially all field crops are hosts although some cultivars are more tolerant of these nematodes than others. They typically will reduce yields of their hosts, but often these losses are minor. They are present in almost all fields where crops are grown. So, really, what’s the big deal?

Plant-parasitic nematodes appear to be increasing in numbers on corn throughout the Midwest and the reality is corn is often rotated with soybeans. Probably the main cause for this phenomenon is the movement away from the traditional at-plant corn insecticides used for corn rootworm control to bio-engineered hybrids. Many of the organophosphates and carbamates used for rootworm control also provided some control of plant-parasitic nematodes including lesion nematodes. However, these new, stacked hybrids do not offer nematode control, so nematode numbers are on the rise.

In soybeans, it has been observed in the MSU Diagnostic Lab that when soybean cyst nematodes are present at high population densities in samples, lesion nematodes are absent or often found in low numbers. If soybean cyst nematodes are absent, up to 1,000 lesion nematodes have been recovered from 1.0 gram of soybean root in some Michigan fields. To manage soybean cyst nematodes, rotation to non-host crops, such as corn, and the use of soybean cyst nematodes-resistant soybean varieties is recommended. However, soybean cyst nematode-resistant varieties are not any more effective against lesion nematodes than susceptible cultivars and these organisms appear to render the soybean cyst nematode-resistant varieties more susceptible to the cysts. Since the tactics against soybean cyst nematodes are limited, this is a concern.

Occasionally, growers note poor performances of soybean cyst nematode-resistant cultivars in fields infested with soybean cyst nematodes. Typically, this is explained as a poor varietal choice as the source of resistance utilized was not the best choice against the type of soybean cyst nematodes present in the field. However, could the poor yield be due to the presence of lesion nematodes and their role as potential resistance breakers? This has yet to be well documented in field situations but does serve as an alternative explanation for locations with lower than expected soybean yields.

When growers become aware of soybean cyst nematodes infestations on their farms, they often acknowledge the infestations but reduce their sampling. Why take the time to sample for nematodes when I know I have them even if the nematode analyses are free (paid by soybean checkoff dollars through the Michigan Soybean Promotion Committee)? If any of you have had these thoughts, I hope to change them. Since the worms are microscopic, the only way to monitor their numbers is to collect soil and root samples on a regular basis. To best assess the performances of varieties, soil samples should always be sampled at planting and at harvest for comparison of nematode numbers. If a soybean cyst nematode-resistant variety is effective, the numbers at harvest should be lower than those at planting. Yield is an important component, but the nematodes should not be ignored. It is important to try to understand why varieties do not perform as advertised so crop loss can be avoided in the future. Is your soybean cyst nematode population increasing in numbers under these resistant cultivars? How about lesion nematode population densities? Soil sampling is the only method to obtain this information.

For questions or concerns about soybean cyst nematodes, lesion or any other nematodes, feel free to contact Fred Warner (517-432-1333) or Angela Tenney (517-353-8563) at MSU Diagnostic Services on campus or Dr. George Bird (517-353-3890) in the Department of Entomology. Soil samples for nematode analyses can be sent to MSU Diagnostic Services, 101 CIPS, Michigan State University, East Lansing, MI 48824-1311. Unless the samples fall under the blanket of the MSPC-sponsored soybean cyst nematodes Sampling Program, there is a $25.00 fee for standard nematode analyses.