Don Hershman, Extension Plant Pathologist
Soybean sudden death syndrome (SDS) caused by the soil-borne, root-rotting fungus, Fusarium virguliforme, is evident in soybean fields across the state. The disease is quite severe in certain fields in west Kentucky, where large areas of fields planted to susceptible and moderately susceptible varieties are affected. SDS has been seen in Kentucky each year since 1985, but incidence varies greatly, depending on the growing conditions. Generally cool temperatures and abundant soil moisture, both of which favor SDS, appear to have set us up for increased incidence and severity of SDS this season. The last time SDS was extensive in KY was in 2009.
Fusarium virguliforme infects roots early in the season and foliar symptoms normally appear during the soybean reproductive stages, as is the case in fields this year. In a more typical year (i.e., hot and dry during July/Aug), a greater extent of SDS is often associated with very early planting; doublecrop beans are rarely affected. However, in a year like this one, planting date associations are blurred and we may find that the disease even ends up developing in doublecrop soybeans as well as full-season beans.
SDS is first evident in foliage as yellow blotches between the veins of leaves in the mid and upper canopy. In most cases, blotches coalesce and result in a yellow and brown discoloration between the veins, but the veins remain green (Figure 1).
Figure 1. Range of SDS foliar symptoms.
In severe cases, symptomatic leaflets will crinkle and eventually fall off (Figure 2), but the petioles will remain attached to the plant. If severe symptoms develop during early- to mid-pod fill, pods may abort and/or fail to fill properly. Up to 85% yield loss is possible in severely diseased areas of fields. If symptoms come in when pods are filled or nearly filled, limited yield loss will occur even when severe foliar symptoms are evident. With SDS, the timing of symptom expression relative to crop stage is the main consideration when assessing probable yield damage caused by the disease.