Nematodes. Some types of these microscopic roundworms can be beneficial, but some are also parasites, causing serious damage to plant roots or even killing crops.
There has been some research published about the ability of cover crops to manage plant-parasitic nematodes, since the roundworms can only move very short distances on their own. But they’ve also raised questions about whether covers can serve as a host.
If you have a field with a nematode problem and plant covers, you should be very particular about which cover crop species you choose to plant in that field, selecting species that do not host nematodes.
Which summer cover crop species are best for keeping nematode populations at lower levels? According to University of Florida Extension there are actually many choices of summer covers, including cowpeas, sunnhemp, sorghum-sudangrass, marigold, jointvetch, velvetbean, hairy indigo, castor, sesame and some grasses. There are several types of winter cover crops that also can help with decreasing nematodes, including cereal rye, hairy vetch, wheat, crimson clover and lupine.
The type of cowpea cultivar used and the type of nematode both play a role in how effective cowpeas are in keeping nematode populations down, while sunnhemp suppresses plant-parasitic nematodes.
Florida studies have shown sorghum-sudangrass does the best job of nematode suppression and cultivar choice is crucial in this situation as well. A fact sheet from the Sustainable Agriculture Research & Education program (SARE) stated that experiments in Maryland with sorghum-sudangrass grown annually as a green manure crop following a nematode-susceptible crop reduced the nematode population as effectively as a nematicide application prior to production of a susceptible soybean cultivar.
Although marigold isn’t considered a cover crop in the Midwest, it works well in warmer climates, as it can suppress 14 different genera of plant-parasitic nematodes. Castor beans are very effective on nematodes as well, but must be planted annually to maintain reduced nematode populations.