Sudden Death Syndrome Of Soybean Confirmed In South Dakota

Sep 07, 2017

By Emmanuel Byamukama

Assistant Professor & SDSU Extension Plant Pathologist

Last week, sudden death syndrome (SDS) was confirmed in Lincoln, Turner and Union counties. Most of the fields found with SDS had very few scattered plants showing symptoms, with the exception of one field in Lincoln County which had a moderate level of SDS (Figure 1). These symptoms can be mistaken for maturing soybeans but a close-up look at the symptomatic plants will show typical SDS symptoms (Figure 1).

Symptoms

Sudden death syndrome is a fungal disease caused by Fusarium virguliforme. This pathogen survives in infested soybean residue and in the soil for several years. Planting in wet, cool soils in the spring promotes infection. The SDS pathogen infects soybean seedlings early in the growing season but symptoms do not appear until the early reproductive stages under cool temperatures and frequent rainfall conditions.

Figure 1. A soybean field with sudden death syndrome in Lincoln County (left) and a close-up of symptomatic leaves (right).

SDS symptoms start as small chlorotic spots on the leaves. These spots quickly expand leading to large yellow blotches which eventually become necrotic (dead leaf tissue) between the leaf veins, leaving vivid green veins (Figure 2). Infected plants have rotted roots. Splitting the stem lengthwise reveals the browned cortex and white pith which is a diagnostic characteristic of SDS. When the soil is moist, blue spores may be noticed on the taproot of infected SDS plants (Figure 3). In order to distinguish SDS from other root and stem rots such as brown stem rot, again split the stem and look for the discolored cortex near the tap root as well as white pith. Brown stem rot does causes the vascular tissues to be brown.

Figure 2. Beginning SDS symptoms (left) and advanced symptoms (right).

Figure 3. Blue fungal growth on the taproot is a sign of Fusarium virguliforme, the pathogen which causes SDS.

Management

There are no in-field treatments for SDS management. Since the SDS pathogen infects the soybean seedlings at planting, managing SDS should start before planting.

SDS is favored by planting in cool wet soils in spring. Therefore plant when soils are warm and not excessively wet.
Some fungicide seed treatments are effective at protecting seedlings from getting infected. Currently an active ingredient that has been tested and found to be effective against SDS is Fluopyram.

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