The relationship between the causal agent of SDS, SCN
Soybean Cyst Nematode (SCN) is an economically important pest of soybean in Wisconsin. It was first discovered in the southeastern part of the state in 1981 and now is found in over 90 percent of the state’s soybean acres. It is caused by the soybean cyst nematode, a non-segmented roundworm that inhabits the soil. More recently, another economically important disease of soybean, Sudden Death Syndrome (SDS), was first found in southeastern Wisconsin in 2006. A fungus found in the soil called Fusarium virguliforme is the causal agent of SDS.
Soybean Cyst Nematode (SCN): In high-yielding fields or during years when soil moisture is plentiful, profoundly visible symptoms of SCN are rarely seen. Subtle symptoms include uneven plant height, a delay in canopy closure, or early maturity. Severely infected plants appear stunted with yellow foliage, and canopy closure may be delayed or not occur in affected areas. Management of SCN should begin by sampling soil to confirm the presence of the nematode. For a detailed description about sampling for SCN in WI, see the pamphlet titled Soybean Cyst Nematode Sampling and Testing in Wisconsin. Additional management should also include an integrated plan where crop rotation and resistant cultivars are be used. Rotating to non-hosts of SCN can help reduce SCN populations in soil. Cultivars resistant to SCN should be planted when numbers of SCN are above suggested thresholds, and sources of resistance (e.g. Peking vs. PI 88788) should be alternated in fields with high populations. When SCN numbers are below threshold, rotating with resistant and susceptible varieties can slow the increase in populations of SCN that can overcome common types of resistance available in commercial soybean cultivars. Cultural practices such as managing weeds, providing adequate fertility, amending soil pH to at least 6.5, and improving soil moisture through tillage and supplemental irrigation can reduce plant stress and help plants deal with SCN populations.
Sudden Death Syndrome (SDS): Symptoms of sudden death syndrome are expressed as yellowing and necrosis between the veins of leaflets. Veins of symptomatic leaves will remain green. Leaflets will eventually curl or shrivel and drop off with only the petiole remaining. Management of SDS includes a combination of strategies. Most importantly, SDS-resistant cultivars should be chosen whenever possible. If SDS and SCN are both problems in a field, choosing a variety with the best resistance/tolerance to both will be beneficial. Planting into cool, wet soils typically seen early in the season can favor infection by the SDS fungus. Delaying planting can reduce the risk of infection, but remember that yield loss will occur from delaying planting too long. Improving soil drainage and reducing compaction can help reduce levels of SDS. Crop rotation can be useful to manage other diseases of soybean; however, research has demonstrated that crop rotation does not significantly reduce levels of the SDS fungus in the soil. Even after several years of planting corn in a field, the SDS fungus can survive on corn plants and corn kernels can harbor the pathogen. In addition, the SDS fungus can survive in soil for long periods of time as specialized, thick-walled spores. For more information about SDS, consult the Wisconsin Farm Fact Sheet XGT1015 titled Sudden Death Syndrome of Soybean.
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