With many agronomic challenges, the gist of any thoughtful response is usually rotation, rotation, rotation. Change up your management strategy every year, so that the pest or weed doesn’t get habituated to whatever you’re doing. When it comes to nematodes – unsegmented, microscopic round worms that live in the soil and can be plant and animal-parasitic – the answers are much the same. Although “rotation” can be a big part of the answer, there’s a common belief that that is synonymous with “cover cropping.”
Soybean cyst nematode (SCN) is a common pest in soybean fields, but also infects varieties of beans, peas, birdsfoot trefoil, chickweed, clover, lespedeza, vetch, and cowpeas. This nematode injects a compound into root structures, altering the root cell biology. The worms embed in the tissue and after a few weeks, they grow so large that they rupture out of the tissue they are feeding on. With high populations of SCN, symptoms may become visible. Otherwise, above-ground symptoms may go undetected, or may manifest as a stunted, chlorotic appearance that can be mistaken for compaction, nutrient deficiencies, drought stress, herbicide injury, and other plant diseases. SCN is a stealthy threat, and can feed off of roots for quite some time before anyone notices above-ground crop damage. Meanwhile, it can cause a yield loss of 30-40 percent with no visual symptoms (above-ground symptoms usually come well after root symptoms are widespread), and costs about $1 billion in total losses annually. After infection of the plant’s root, the first symptoms will be root stunting, discoloration, and fewer nitrogen-fixing root nodules. Females can be seen with the naked eye. In the field, the infected plants will often form a circular or oval area. The only way to make an accurate diagnosis is to observe the adult females and cysts on the roots. SCN populations can also be monitored in soil tests. Egg counts over 2000/half cup of soil are a cause for concern. If they keep increasing on a non-host, you know they have adapted.
SCN is not very mobile in soil, so the soil and nematodes have to be moved by machinery, vehicles, tools, wind, water, animals, or workers for the infestation to spread. SCN is very adaptable, however, building resistance and becoming more aggressive in a field over time if a variety of control strategies are not used. Infestation is usually cumulative over time if no action is taken, and even a year of using an SCN susceptible variety or good host crop in a problem field can set you back several years.
Root-knot nematode (RKN) is a cousin of SCN and a similar threat to agricultural and horticultural crops, with an even wider host range. RKN establishes feeding sites within plant roots, causing enlargement of root cells (the visible “knots” or swollen areas that form on the roots). Both SCN and RKN can be hosted by a range of weed species, such as henbit and deadnettle, which makes good rotation and weed management even more critical.
Do cover crops work? It depends
SCN is still one of the most yield-suppressing pathogens, in spite of rising cover crop use in recent years. Be careful when planning your IPM strategy. One of the best assets cover crops bring to the table is diversity, which in turn promotes below-ground diversity in soil fauna. Many cover crops can be a host to SCN – largely legume cover crops. When it comes to SCN and other harmful nematodes, grass crops are often the least likely to be hosts, while almost all legumes host some populations of nematodes. The most basic rotation recommendation for SCN is to incorporate non-hosts like corn, wheat, and alfalfa, as well as SCN-resistant soybean varieties, into the rotation, but be sure to rotate these as well. The less chance the pest has for adaptation, the better.
The right cover crops can suppress nematodes directly and indirectly, using four main mechanisms (these apply to many pest-suppressing cover crops) –
- Production of biofumigant/nematicidal compounds – crops in the brassica family produce methyl-isothiocyanates as they decompose.
- Trap crop – Juveniles that hatch in a cover crop’s roots are trapped and die.
- Pest starver – non-host
- Induced hatching of juveniles – The root exudates from cover crops that grow in the “off” season stimulate hatching of SCN juveniles in fall or spring. Hatching at these cooler times when there is no food source means they die of starvation.
- Producing inhibitory allelochemicals – either while living or decomposing.
- Foster biological control – Cover crops have a long-term effect of enhancing soil health, which increases nematode diversity, including predatory nematodes they prey on plant-parasitic nematodes.
The data on cover crop suppression of SCN is inconsistent, but we know that many common cover crop species, especially cereals, are not good hosts, and may work for suppression. Cereal rye has shown good potential for reducing populations. Rye planted earlier in the fall is more successful in reducing SCN populations, as it has more growing time to produce allelochemicals. Annual ryegrass is also a good option. The Illinois Soybean Association reports that no-tilling soybeans into wheat stubble can be effective. As far as summer annuals, sorghum-sudan has shown to be a non-host across various nematodes, as have many species of brassica.
It can be difficult to interpret studies and gauge effectiveness of various cover crop species, because cooler temperatures mean that SCN isn’t very active in the typical winter annual cover crop growing window to begin with. Greenhouse studies on SCN are likewise hard to translate to the field, because the hot temperatures activate the pathogen at a time when it may not normally be active.
Complicating matters further, there are regional differences in the host range, so SCN might be able to feed and reproduce on a host in one region but not another. This leads to mixed information about what is a good host, but legumes are certainly a common theme for good hosts. Almost all legumes are considered hosts somewhere in the US, and the pea and vetch families tend to be universally better hosts. For example, Ohio State University lists alsike clover, Birdsfoot trefoil, green beans, dry beans, common and hairy vetch, cowpeas, crimson clover, lespedeza, peas, white and yellow lupine, and sweetclover as good hosts. The list from other states will vary, and even different varieties of the same species will have varying susceptibility levels.
This brings us back to the main point – the key to managing SCN and most agronomic challenges is diversity and rotation, not a cover crop cure-all. Rotate not only your main crop, but also your cover crop. Most cover crops will also provide food for beneficial nematodes. Suppressive crops can reduce populations enough that you can grow host crops reasonably well.
In addition to cover crops, other control approaches can be used, including resistant varieties and chemical controls, such as a seed treatment nematicide. Fallow is also somewhat effective, since it provides no host, but leaves all the risks associated with bare soil, such as erosion and loss of vital biodiversity in soil life that also relies on living roots for food and shelter.