Last week, symptoms of sudden death syndrome (SDS) began to appear in April 15-planted soybeans at the University of Illinois's Northwestern Research Center in Warren County, according to a U of I Extension educator.
Angie Peltier explained that weather during the 2015 growing season has been favorable for the development of SDS: cool, moist soils after planting followed by frequent rains. Symptoms began appearing approximately 3 weeks earlier than in 2014, she said.
Although the fungus that causes SDS (Fusarium virguliforme) infects roots of soybean seedlings very early in the growing season, foliar symptoms don't typically appear until after soybean plants reach reproductive growth stages. Peltier said foliar symptoms begin with a yellowing of the tissue between leaf veins. This tissue then dies, becoming brown in color with only the leaf veins remaining green. Leaves eventually fall off, while petioles remain attached to the main stem. The earlier that symptoms develop and leaf drop occurs, the greater the potential for yield loss.
"Although foliar symptoms of SDS can be easily confused with those of another disease—brown stem rot—one need only split the plant lengthwise to distinguish the two," Peltier explained. "Brown stem rot causes browning of the innermost stem tissue (pith). Stems of plants with SDS remain healthy. Blueish-white spores of Fusarium virguliforme can sometimes (not always) be seen on the roots of symptomatic plants."
Although the most conspicuous symptoms of SDS occur in leaves, the fungus itself remains in the roots and in the stem nearest to the soil line. Foliar symptoms are caused by toxins produced by the pathogen. These toxins are carried along with water to leaves through the xylem tissue.
"The SDS disease cycle has important implications as far as management is concerned. Infection and colonization have long since taken place and there are no mid-season management tools with which to manage this disease," Peltier said. "Management decisions must be made before the growing season begins."
Peltier explained that the best way to manage SDS is to plant the most resistant varieties. "Soybean varieties vary considerably in their level of genetic resistance." she said.
To provide impartial SDS resistance ratings to help soybean producers more easily compare varietal resistance among seed brands, teams led by Jason Bond of Southern Illinois University and Silvia Cianzio of Iowa State University evaluated more than 500 soybean varieties (MGs 0 to V) from 19 different seed companies. Results from these 2014 check-off sponsored trials are available online. Results from the 2015 trials will be compiled and released in October in time for producers to use when making their 2016 seed purchases.
Research has also shown that SDS may be more severe in fields that also have high populations of soybean cyst nematodes (SCN). Monitoring SCN populations and planting SCN-resistant soybean varieties can also be important components to managing SDS.
Peltier pointed out that the newest tools available for managing this disease are fungicidal seed treatments labeled specifically for SDS. Former U of I Extension plant pathologist, Carl Bradley, and his team conducted several SDS seed treatment trials. In these trials, the active ingredient in ILeVO (fluopyram) showed efficacy against SDS. Other SDS seed treatments on the market are also being evaluated, she added.