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Symptoms and Signs

Sudden death syndrome (SDS) of soybean is typically not detectable on the foliage of plants until after the beginning of flowering. Under rare circumstances younger plants may show symptoms. It is always useful to compare the affected plants with healthy plants of the same field when making disease assessments.

Figure 1a

Figure 1b

Figure 1c

Foliage and stems

Early symptoms of SDS are diffuse chlorotic mottling and crinkling of the leaves. Later, leaf tissue between the major veins turns yellow, then dies and turns brown. This interveinal chlorosis and necrosis is typical for SDS, but these symptoms are not diagnostic by themselves because they can be confused with foliar symptoms of other diseases. Soon after, the leaflets die and shrivel. In severe cases, the leaflets will drop off, leaving the petioles attached. For diagnosis, the lower stem and the taproot must be split. The cortical tissue of a plant with SDS will exhibit tan to light brown streaks, whereas the cortex of a healthy plant will be white. The pith (the central portion of the stem) of an infected plant will remain white or slightly cream-colored.

Figure 2a

Figure 2b

Figure 2c

Roots

If a plant with advanced foliar symptoms of SDS is removed from the soil, root systems will lack vigor when compared to a healthy root system. Roots may also be rotted. If the plants are collected when soil is moist, small, light-blue patches may be visible on the surface of the taproot near the soil line. These patches are blue spore masses of the fungi that cause SDS. As the root surface dries, the blue color will fade, but these blue spore masses, seen in conjunction with the other symptoms mentioned above, are strong diagnostic indicators of SDS.

Distinguishing SDS from similar diseases

Brown stem rot, stem canker, charcoal rot, and chemical burn can all produce foliar symptoms similar to SDS. However, in these other diseases, dead leaflets tend to remain attached to the petioles. Brown stem rot symptoms differ from SDS symptoms by the darkened pith and insignificant discoloration of the cortex. Stem canker is often associated with spore mats within the stem that are visible as longitudinal, net-like striations. Charcoal rot gets its name from the microsclerotia that develop within stem tissue and that resemble a fine layer of charcoal dust. No signs of pathogens are found when chemical burn is responsible for foliar symptoms and roots of affected plants would most likely appear healthy.

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