Sudden Death Syndrome in Soybeans?

It is that time of year again when fields in southern Ontario begin to display typical symptoms of sudden death syndrome. Although sudden death syndrome has been a problem in the U.S. for over 40 years, it is unfortunately becoming more frequent in some fields in the southwestern.

The pathogen infects soybean plants early but symptoms generally do not develop or become noticeable until the mid-pod reproductive stage of soybean development. Sudden death syndrome is most problematic during year's that are cool/warm and wet. This year's environmental conditions have been ideal for the disease. Although the disease is most often found in field's that are infested with soybean cyst nematode, the disease can occur without SCN being present. By penetrating the root, the nematode leaves openings or wounds that allow the fungus that causes sudden death syndrome easy access to the internal root tissue. The result is increased disease. At present the most effective method to manage sudden death syndrome is the use of tolerant varieties. Check with your seed company for variety tolerance ratings.

Sudden Death Syndrome is a root rot disease caused by a more virulent or aggressive strain of the common soil-borne fungal pathogen Fusarium virguliforme formerly known as Fusarium solani f.sp. glycines. Besides typical root rot symptoms, infected plants will begin to show distinct yellow speckling or spotting between the veins (interveinal chlorosis). As the disease progresses these areas become larger and in severe cases the entire area between the veins becomes brown (necrotic) and dry. The leaves curl from the outer margin. These brown areas eventually disintegrate (fall out) leaving only the veins on the leaf. All through this process the veins remain green.

The next symptom is often premature defoliation and death of the plant. In most cases, these dead plants the petioles (leaf stalks) remain attached to the stem.

Cutting the root and the stem will expose a light brown or gray discolourization of the cortex tissue in the tap root and the stem. This discolourization will extend from the roots up through the stem. Although the leaf symptoms may resemble brown stem rot, in sudden death infected plants the pith (centre) of the stem remains white whereas brown stem rot infected plants have a brown and disintegrated pith.

Since infection occurs early in the growing season, a fungicide seed treatment may reduce early infection but to date none of the available seed treatments have excellent SDS control.  As a result, other management options include crop rotation with corn and small grains will reduce the incidence of disease.  The disease occurs often under reduced tillage the incorporation or removal of infested residue can help reduce your risk to the disease.  Seed companies are making progress and more resistant or tolerant varieties are becoming available.

Figure 1. Yellow spots between veins

Figure 2. Interveinal Browning

Figure 3. Leaf Curl

Figure 4. Leaf Stalks Remain Attached

Figure 5. Browning

Figure 6. Discoloured Cortex