No. 22/August 29, 1997
Sudden Death Syndrome
Brown stem rot (BSR) and sudden death syndrome (SDS), two mid- to late-season soybean diseases, have prompted numerous calls from throughout the state. An earlier article in the Bulletin had noted that, although there were no reports of these two diseases yet, we expected to find them if weather conditions were to change to favor the fungal pathogens. Unfortunately, that has now happened. With the recent rainy spells and a return to cooler weather, both pathogens have become active. Both cause leaf discoloration and loss of yield, depending on time of infection and the stage of crop growth. BSR is more common in northern Illinois. SDS can be found statewide, although it is most commonly seen in the southern third of the state.
Sudden death syndrome (SDS), caused by a strain of the soilborne fungus Fusarium solani generally appears about midsummer in soybeans with high yield potential, usually after blooming. The disease can result in minor or severe yield loss, depending on when it develops. SDS is identified by the appearance of small, scattered yellow spots or blotches, usually on the upper leaves. These spots enlarge and merge, and the tissues turn brown between the veins; however, the veinal tissues remain green. Leaflets may curl upward or drop prematurely, leaving the petioles firmly attached. Severe foliar symptoms give affected areas in a field a tan to brown cast and may be the first evidence of the disease. Flowers and pods may abort and pods drop or not fill. The first pods to set may have a few beans in them that remain small. Later pods may not fill or may have immature green seed. One characteristic of SDS is that the interior of the stem (the pith region) remains white. There may be a slight gray-brown discoloration of the vascular system just inside the outer "bark" of the stem, but the pith remains white. If the pith is discolored, it may indicate the presence of brown stem rot. Root symptoms preclude foliar symptoms and result in deterioration of the topmost lateral roots and of nitrogen-fixing nodules. Fields in which the disease is present are likely to develop SDS in subsequent years, although there are no accurate methods of assessing possible disease levels.
The Fusarium fungus overseasons as thick-walled chlamydospores or mycelium in crop debris or in soil. SDS is affected by weather conditions. The disease is more severe during cool, wet growing seasons. It is commonly found in association with soybean cyst nematodes (SCN) and in lower areas of fields. Nematodes are believed to act as a stress factor, rather than being directly involved with the disease. However, work in Mississippi has shown that SCN can act to spread the fungus. This research demonstrated that the Fusarium fungus was present both on and in the cysts of soybean cyst nematode. Therefore, direct or indirect movement of the nematode could spread the fungus to new areas. The disease tends to be most severe on well-managed soybeans with a high yield potential. However, tillage and rotation practices seem to have little impact on this disease.
- Grow well-adapted, high-yielding varieties in a warm, well-drained, fertile soil. Maintain balanced soil fertility, based on a soil test.
- Control other diseases, weeds, and insects.
- Although SDS is not seed-transmitted, seeds from infected plants are small in size and tend to produce weaker seedlings than those from healthy plants. Therefore, do NOT save seed from SDS-infected areas.
- Crop rotation, although not consistent in greatly reducing levels of the Fusarium fungus, is definitely beneficial in reducing the buildup of other pathogens (especially nematodes) that may weaken the plant.
- Sanitation (for example, cleaning tires, combines, and other equipment of soil and crop debris), although time consuming, helps to reduce spread of the SDS fungus as well as other soybean pathogens.
H. Walker Kirby,Department of Crop Sciences, (217)333-8414