SDS is a fungal disease caused by Fusarium solani f. sp. glycines. Although the symptoms of SDS develop after mid-July in Iowa, infection by the fungus occurs as early as the seedling stage when soils are cold and wet. Research has shown that low soil temperatures favor fungal infection and that fields planted early have a higher SDS risk than later-planted soybeans.
SDS symptoms are fairly easy to identify
Although the fungus colonizes the roots early in the season, leaf symptoms rarely appear until mid July. Leaves of infected plants initially show scattered yellow spots between leaf veins (Figure 1). The irregular shape of the spots and the green areas around the leaf veins give SDS its characteristic pattern. The tissue in these spots starts to die and enlarges to form brown streaks between the veins, called interveinal necrosis (Figure 2). Only the midvein and major lateral veins remain green. Leaflets drop eventually but the petioles remain on the stem.
Another symptom of SDS is rotted roots. Diseased plants have deteriorated taproots and lateral roots. The root cortex is light gray to brown, and the discoloration may extend up into the stem 2 inches above ground. Sometimes bluish fungal colonies can be seen on the root if soil moisture is high (Figure 3).
Foliar symptoms of SDS look similar to some brown stem rot (BSR) strains and sometimes the two diseases are misdiagnosed. A good way to distinguish between the two is to check roots of diseased plants. Plants with SDS will have root rot, while BSR does not cause root rot.
Management of Sudden Death Syndrome
If you find light SDS in your fields, the disease may or may not develop into a severe problem in the future. If the disease is light (no hot spots with severe defoliation), scout for the disease in the next soybean crop.
If the disease is already causing damage in your fields, avoid planting susceptible varieties. Many SDS-tolerant varieties are available in Iowa.
The fungus can overwinter in cysts of the soybean cyst nematode (SCN), increasing its ability to survive from season to season. Thus, the presence of SCN may favor development of SDS. Management practices to reduce soybean cyst nematode populations are thought to also delay onset of SDS.
Two years between soybean crops and maintaining good crop nutrition have been shown to reduce incidence and severity of SDS.
SDS severity has been associated with compacted soil. Therefore, deep tillage may also reduce severity of SDS on headlands and other parts of the field where the soil is compacted.