Sudden Death Syndrome -

Sudden death syndrome (SDS) has spread from southern soybean areas to many of the major soybean-producing states in the Corn Belt. Illinois has been most impacted by the disease. SDS is caused by a specific strain of a common soil fungus known as Fusarium solani. This fungus infects soybean roots and produces a toxin that damages the leaves.

Leaf symptoms of SDS first appear as yellow spots, usually on the upper leaves, in a mosaic pattern. The yellow spots coalesce to form chlorotic (lacking green pigment) blotches between the leaf veins. As these chlorotic areas begin to die, the leaf assumes a distinct appearance, with yellow and brown areas contrasted against a green midvein and green lateral veins. Rapid drying of necrotic (dead tissue) areas can cause curling of affected leaves. Leaves drop from the plant prematurely, but leaf petioles remain firmly attached to the stem. Flower and pod abortion are common, resulting in fewer pods and seeds produced.

Root Symptoms - SDS limits root development and deteriorates roots and nodules. A blue coloration (spores) may be found on the outer surface of tap roots. Splitting the root reveals that the outer cells have turned a milky gray-brown color while the inner core, or pith, remains white.

The Fusarium solani strain that causes SDS survives in crop debris and as mycelia in the soil. The organism enters soybean roots through wounds caused by SCN or insect feeding, or mechanical injury. The fungus then colonizes the soybean root system. A toxin produced by the fungus and translocated throughout the plant is responsible for above-ground symptoms.

Like other soil-borne root rots, SDS often appears first in certain spots in the field, such as low, poorly drained or compacted areas. SDS severity is highly dependent on environmental conditions, time of infection and other stresses on the soybean crop. Higher incidence of SDS often occurs when soybeans have been exposed to cool, moist soil conditions early in the growing season.

The appearance of SDS symptoms is often associated with weather patterns that bring cooler temperatures and significant rainfall to an area during flowering or pod-fill. SDS symptoms are usually more severe if soybean cyst nematode (SCN) is also problematic in the field. SCN increases the stress on the soybean plant, and also provides wounds through which the SDS pathogen can enter the roots. Scientists have also discovered that F. solani can be carried in SCN bodies.

SDS has proven to be a difficult disease to manage. Currently, there are no soybean varieties that show total immunity to SDS. Likewise, there are no fungicides that offer protection against this fungal pathogen. Rotation has not been particularly helpful because the causal organism can persist in the soil for many years. However, there are several cultural practices that can help growers minimize the damage due to SDS.

Growers should carefully scout and document both the presence and severity of SDS in their fields, then employ appropriate management strategies on a field-by-field basis. First among these is selecting tolerant soybean varieties that show a reduction in SDS severity. Then, consider delaying planting, improving field drainage and reducing compaction, and evaluating tillage systems. Reducing other plant stresses can make soybeans less vulnerable to SDS attack. These include herbicide stress, nutrient deficiencies, high pH and SCN. Eliminate as many of these stresses as possible to reduce the impact of SDS.