By James Hoorman, Hoorman Soil Health Services
Sudden Death Syndrome (SDS) and soybean cyst nematode (SCN) damage are often linked together. SDS is a soil borne fungal pathogen (Fusarium virguliforme) that invades the roots and lower stems of soybeans producing a toxin. SDS can devastate soybean fields causing aborted flowers and yellow dying plants. SDS has two major phases. In the first phase, it attacks the roots then in the second phase, it attacks the leaves causing leaf scorch. SDS infection occurs early in the season and then the SDS symptoms show up later in the season. SDS and SCN symptoms are more prominent in hot dry years.
Foliar SDS symptoms include small to pale green leaves early on with small circular spots in the late vegetative stages to early reproductive soybean stages. The area between the leaf veins turn bright yellow then brown as the disease progresses. When the infection gets severe, on roots, blue fungal masses can be seen. SDS is common on sandy soils, top of hills and knolls, but also on plants under stress such as on poorly drained or compacted soils.
The USA annual SCN loss is estimated to be $1 billion dollars per year. SDS is carried by SCN which are like tiny worms that feed on the soybean roots. The SCN life cycle is 25-30 days with each adult laying up to 500 eggs. Severe soybean damage occurs when SCN reach 10,000 nematodes per plant, becoming yellow and stunted. Common weed hosts for SCN include henbit, purple dead nettle, and mulleins.
Farmers have to use a combination of crop management practices to avoid SDS and SCN because there is no single practice that stops SDS or SCN completely. The first step to reduce SDS and SCN is crop rotation. Non-host crops like corn and wheat reduce SCN numbers in soybeans. Depending upon the severity, it may take 2-3 years rotating away from soybeans.
Other strategies farmers use include finding soybeans traits that are highly resistant to SDS and SCN. Most soybean seed companies have a SDS rating on soybean varieties, but none are totally resistant to SDS or SCN. Newly released herbicide soybean traits often lack resistance to SDS or SCN.
Farmers also use nematicides to control SCN, especially seed treatments that kill all nematodes. Most nematodes do no harm to soybeans. This strategy tends to kill everything, good and bad. Often beneficial organisms that feed on pests are harmed when toxic nonselective pesticides are used as seed treatments. Natural enemies of nematodes include mites, collembola (springtail), flatworms, protozoa, and even other predatory nematodes. Soybean fungal seed treatments work to reduce the SDS Fusarium pathogen. Other strategies farmers can use include improving drainage, minimizing compaction, maintaining good crop nutrition, correcting soil pH, and staggering planting dates.
SCN is most damaging yield-limiting soybean disease in the USA. SCN is soil dispersed with equipment, water, and wind. Yet many prominent soybean specialists recommend full tillage to control many crop diseases with the goal of burying crop residue. All these practices contribute to lower SOM, reduced water infiltration, more erosion (water and wind), soil dispersion of SCN by equipment, plus higher soil compaction. Excess tillage increases SCN. Long-term NT plus cover crops do the opposite. Perhaps that is why I seldom see NT and cover crop farmers complain about SCN or SDS. Some farmers who bought or leased farmland that have high SCN or SDS levels have reduced levels after several years of using long tern no-till and cover crops.
Planting annual ryegrass or cereal rye as a cover crop has been shown to reduce SCN numbers up to 80% if the cover crop is planted early in the fall when soil temperatures are above 500F. SCN come out of their cysts when soil temperatures are above 500F, and for a short period of time in the fall, they cannot recognize a host plant from a non-host plant. If these grasses are planted early, the SCN are tricked into coming out of their protective cyst cycle, and once out, they do not survive the winter.
Fighting SDS and SCN is a long-term process. As most agronomist have said, there is no silver bullet. Cover crops help poorly drained soils improve water infiltration but also improve water drainage by improving soil structure. The improved soil structure also means that soil compaction is minimized. The addition of soil organic matter on sandy soils and clay soils improves nutrient retention but also makes soil more resilient. Healthy plants can overcome different stress in their life cycle. Improving soil health helps with all these problems and tends to make SDS and SCN less of a problem.