Soybean Sudden Death Syndrome – (Andreas Westphal, Scott Abney, and Gregory Shaner)

The growing season of 2007 has been uncharacteristically dry. Soil conditions were favorable for field operations in late April and early May. Planting operations progressed mostly in proper seedbeds. So far, based on previous years’ experience, conditions have been unfavorable for SDS this year. Severe SDS is often associated with excess soil moisture as plants move into reproductive stages. However, in fields with a history of SDS the disease may develop in the absence of wet soil during this period of plant development. Even in this dry year, soybean fields should be monitored for presence of SDS.

Sudden death syndrome is caused by the soil-borne fungus Fusarium virguliforme (previously—Fusarium solani f. sp. glycines). The fungus occurs in soybean fields in nearly all areas of Indiana, and can cause SDS when conditions are favorable. Fusarium virguliforme invades soybean roots early in the season, but it is only at mid-season when plants are in reproductive growth stages that aboveground symptoms of SDS occur. Heavy rains during reproductive stages are a predisposing factor for SDS, but symptoms have appeared this year in some fields that have not received heavy rains. During reproductive stages of soybean the fungus can produce toxins in the root system, which the plant translocates to the leaves, where they induce foliar symptoms. Symptoms of SDS have appeared this year later than normal, and so far primarily in highly susceptible soybean varieties growing in fields with a history of SDS.

In affected plants, leaf tissue between the major veins turns yellow, then brown. Soon, the leaflets die and shrivel. In severe cases they drop off, leaving the petioles (leaf stalks) attached. Stem symptoms offer the best clues for diagnosis of SDS and for distinguishing this disease from brown stem rot. When split, the lower stem and taproot of a plant with SDS will exhibit a dark cortex, but white to tan pith. Brown stem rot may cause similar foliar symptoms, but the leaflets tend to remain attached to the petioles. Plants with brown stem rot have a dark pith, but the cortex is not much discolored. If a plant with symptoms of SDS is dug up when soil is moist, there may be small, light-blue patches on the surface of the taproot. These are spore masses of the SDS fungus. As the plant dries, this color will fade, but when it is seen, in conjunction with the other symptoms mentioned above, a diagnosis of SDS is strongly indicated.

Early planting into cool soils favors SDS. It is not possible to predict how severe or widespread SDS will be this year, but growers should be on the alert for appearance of symptoms over the next 2 or 3 weeks. Plants killed early by SDS may produce no seed. Plants that develop symptoms later may produce almost as many pods and seed as healthy plants, but seed may be smaller.

There is no remedy for plants that develop SDS. Fungicides will have no effect on this disease. Growers should keep records of which fields develop SDS. Even better, growers should map problem areas within fields. SDS rarely affects entire fields. More often, the disease shows up in patches—low areas or areas of compacted soil are often the first to show symptoms. After the first appearance of symptoms, patches may enlarge and more patches may develop as the season progresses. Identification of problem fields can be useful for future planting decisions. A grower should choose a soybean variety that is less susceptible to SDS for planting into fields with a history of SDS. Although there are no highly resistant varieties, some are much less susceptible than others. Late planting may reduce the risk for SDS, but this may not apply when May is unusually cold.

Although Fusarium virguliforme alone can cause SDS, there has long been an observed association between this disease and presence of the soybean cyst nematode in a field. Recent studies at Purdue have shown that symptoms become much more severe when both the fungus and the nematode are present in a field. If a field develops symptoms of SDS and the grower does not know the status of soybean cyst nematode in that field, soil should be tested to determine population levels of this pest.

Leaf with typical interveinal necrosis

Canopy view of diseased and healthy leaves

Entire plant with some of the leaves dropped and only the petioles remaining

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