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Sudden Death Syndrome, Stem Canker, and Fusarium Wilt in Soybeans

It is that time of year again when late season soybean diseases such as sudden death syndrome, stem canker, brown stem rot, fusarium wilt develop. Although these diseases do occur at other times of the season, they are most apparent when the soybean crop enters the reproductive stages of development. During this time most of the resources of the plant begin to shift from vegetative (leafs, stems, roots, etc) growth to producing and filling the pods. Due to this shift in resources, the plants natural defense mechanisms are reduced (especially under stressed conditions) thus allowing many of these later season disease causing organisms an opportunity to begin the infection process.

The following are some diseases that have begun to develop over the past few weeks. Knowing which diseases are present can help you in future variety decisions.

Sudden Death Syndrome

Sudden Death Syndrome is a good example of how the change in soybean development increases susceptibility or allows for symptom expression. Sudden Death Syndrome is a root rot disease caused by a more virulent or aggressive strain of the common soil-borne fungal pathogen Fusarium solani. Although infection often occurs during the first 6 weeks after emergence, symptoms generally do not develop or become noticeable until the mid-pod reproductive stage of soybean development. Even though, the disease is most often found in field's that are infested with soybean cyst nematode, SDS can occur without SCN being present. By penetrating the root, the nematode produces openings or wounds that allow the fungus responsible for 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.

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Figure 1.Sudden Death Syndrome Symptoms

Symptoms

Besides typical root rot symptoms, SDS 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, the petioles (leaf stalks) on these dead plants 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.

Stem Canker

Stem Canker may cause seedling damping off and wilt but commonly affects soybean plants after flowering. Stem canker symptoms are often described as a general yellowing of the top leaves of the plant with dark reddish-brown sunken cankers formed at the lower nodes. The lesion may extend several inches, often on one side but does not usually extend down as far as the soil line. In severe cases the lesion may cover the entire length of the stem. A cross section of the stem will reveal a slight browning at the nodes at first followed by complete disintegration of the stem in severely infected plants. The sudden wilting of the plants and the stem canker can be confused with Phytophthora root rot.

Figure 2. Stem Canker lesion starts on the node

The fungus can also cause a stem or tip dieback late in the growing season. In this case, the upper four to six internodes or branches of the plant become dark brown and as the name implies the top dies. Soybean plants may become susceptible as the plants go through physiological changes due to flowering.

The fungi that cause sudden death syndrome, brown stem and stem canker survive for long periods in crop debris (residue) in the soil. Brown stem rot infects early in the growing season but does not appear until a month before harvest. Stem canker prefers moderately warm, wet weather and occurs from mid-July to maturity. Plants infected with sudden death syndrome begin showing symptoms from flowering to maturity and prefer cool, moist soil conditions. Well-fertilized or vigorously growing fields are most likely to show the sudden death syndrome symptoms.

Fusarium Wilt

Fusarium Wilt, as the name suggests, causes a wilting of the soybean plant. This wilting is often misdiagnosed as Phytophthora root rot or Phytophthora wilt. From a distance the affected plants do look to have Phytophthora root rot. Affected plants have a wilting of the stem tips and the upper leaves are scorched. The middle and lower leaves can turn yellow or have pale (dull) yellow spots. In severe cases the leaves will dry up and drop prematurely leaving the petiole behind. The leaves usually remain attached to plants killed by Phytophthora root rot. There also is no evidence of a stem lesion or external decay does not go above the soil line as with Phytophthora.

A lengthwise cut through the roots and stem will reveal a browning of the vascular tissue and pith. This may be confused with early-season brown stem rot. Fusarium infested roots often have red, orange or white mycelium visible.

Fusarium wilt and root rot and Rhizoctonia root rot are most problematic when soybeans are under water and root stress. Stress can interfere with the normal activity of the plant's root system and therefore, will affect growth and potentially increase root diseases.