Sudden Death Syndrome Increasing in South Dakota Soybean Fields

Febina Mathew
Assistant Professor & Oilseeds Plant Pathologist

Fig. 1. Initial leaf symptoms of sudden death syndrome. Note the light green to yellow spots on the leaves caused by sudden death syndrome pathogen. Photo credit: Emmanuel Byamukama

Sudden death syndrome (SDS) has been quietly increasing in South Dakota soybean fields in recent years. The heavy rains following planting in early June combined with late season rain have provided ideal conditions for the disease to develop. This year, SDS was observed in several soybean fields in much of the Midwestern states including Minnesota, Michigan, Wisconsin, Iowa, Illinois, and Nebraska. It has not been confirmed in North Dakota yet.

In a recent survey, 209 soybean fields covering 24 counties in South Dakota (Union, Clay, Yankton, Lincoln, Turner, Codington, Clark, Spink, Brown, Marshall, Roberts, Kingsbury, Miner, Sanborn, Davison, Hanson, McCook, Minnehaha, Lake, Deuel, Hamlin, Brookings, Grant, and Moody), were scouted for diseases. The fields were arbitrarily selected and soybeans were at R2-R5 growth stages in these counties. We suspect SDS to be the most prevalent of the soybean diseases. Based on the visual evidence (light-blue coloration on the outer roots, yellow spots on the leaves between veins, reddish-brown discoloration inside the lower stem and root), about 30 fields in 18 counties seem to be positive for SDS. However, only a few of the more infected soybean fields will have low to moderate yield losses from SDS. In 2013, SDS was confirmed in eight counties, of which only four counties (Lake, Brown, Brookings and Yankton) were positive for SDS in both  2013 and 2014. In order to confirm the SDS pathogen (Fusarium virguliforme), symptomatic soybean plants were sampled from each of these fields and pathogen identification will be established using traditional and molecular techniques.

Like other root rots, SDS often appears first in patches in the field, such as low, poorly drained or compacted areas. The disease incidence can pick up when soybeans are exposed to cool, moist soil conditions early in the growing season. Although SDS affects soybean plants during the vegetative stages, symptoms usually do not appear until the reproductive stages of crop development. SDS begins as small, bright, pale green to yellow circular spots on the top leaves during flowering or pod-fill (Fig. 1). As the disease progresses, the tissue in these spots starts to die and enlarges to form dark brown spots between the veins (Fig. 2).

Fig.2. Advanced sudden death syndrome symptoms. Notice the brown blotches between the veins on the lower leaf. Photo credit: Emmanuel Byamukama

SDS is often, but not always, associated with high levels of soybean cyst nematode (SCN) infestation. The SDS fungus can also use the protective environment inside SCN cysts to survive the winter. Thus, it is important to continue to make SCN sampling and management a priority.

One of the driving factors for SDS development this season was significant rainfall during the late-vegetative and early reproductive stages. For example, the precipitation total in Lincoln County for the months of June and August were 13.7 and 22.73 inches respectively (The 30-yr mean was 4.01 and 16.64 inches for June and August respectively; National Weather Service/National Climatic Data Center (NWS/NCDC) Co-Operative Observing station (COOP) provided by the South Dakota State Climate Office).

SDS management can be accomplished through an integrated approach: use of resistant cultivars, planting in well drained warm soils, crop rotation, managing SCN, and harvesting corn cleanly (SDS pathogen can survive on corn kernels). Foliar applied fungicides are not effective against SDS.

Acknowledgement

This survey was supported by the South Dakota Soybean Association and South Dakota Soybean Research & Promotion Council. We thank the supporting staff and students in the labs of Dr. Mathew and Dr. Byamukama for their assistance. We also thank Dr. Dennis Todey and team (Department of Agricultural and Biosystems engineering, SD Climate & Weather) for their help on obtaining weather data.

Source : SDSU