Issue No. 18, Article 6/July 28, 2006
Sudden Death Syndrome--What's Up This Year?
First an introduction: my name is Loretta Ortiz-Ribbing, and I am the new University of Illinois Extension specialist for crop systems, located at the Macomb Extension Center. My background is diverse, including a B.S. in agronomy (crop protection), an M.S. in forestry (soils), and a Ph.D. in plant pathology. I have experience in agribusiness and worked for Velsicol Chemical Co. in herbicide product development. I also have experience working with turfgrass pathology and management from the University of Illinois. My previous Extension experience includes two temporary positions with the U of I: Extension specialist in vegetable pathology, where I had statewide responsibilities for vegetable pathology and maintained the Illinois Fruit and Vegetable Newsletter, and as an Extension educator for IPM in the east-central regional office, where I had IPM responsibilities for 13 counties for agronomic and horticultural crops. Prior to my current position, I was the agriculture and natural resources extension educator for Purdue University in Elkhart County, Indiana. My postdoctoral research focused on the use of native fungi as bioherbicides, and I currently am working on a project looking at organisms that control common waterhemp and other pigweed species. While my major focus is pathology and IPM, I look forward to using my diverse background to assist clients and develop applied research programs in the western region of Illinois. Now on to sudden death syndrome (SDS).
Typically, foliar symptoms for SDS show up in late July and early August; however, in southern Illinois, foliar symptoms appeared in some sentinel plots almost a month ago. It will be interesting to see the impact SDS has in these early symptomatic fields.
The soilborne fungus Fusarium solani f. sp. glycines, which is the causal organism of SDS on soybeans, infects soybean roots. This pathogen stays on the root system of the soybean plant; however, the fungus produces a toxin that is translocated upward in the plant, causing foliar symptoms.
The foliar symptoms produced by SDS begin as chlorosis and necrosis of the interveinal tissue of soybean leaves, which then enlarge, forming yellow and brown areas between the green midvein and green lateral veins. Infected plants in the field prematurely turn yellow and then brown, whereas healthy plants remain green. Other symptoms include rotting of roots, necrosis of the crown, discoloration of the vascular tissue in roots and stems, premature defoliation of the soybean plant, and abortion of the flowers and pods. When leaves drop off the soybean plant, they often drop from the top, leaving the petiole attached to the stem.
Foliar symptoms caused by SDS infection.
The foliar symptoms of SDS seen in the field are similar to those of brown stem rot, but internal symptoms differ, in that with SDS there is no pith discoloration. Some uniform reddish brown vascular discoloration can occur with SDS, but without a streaking pattern. Leaf symptoms on plants with stem canker can be confused with SDS; however, soybeans with stem canker have cankers on the lower stem, and plants with SDS do not have cankers. Information on stem canker and brown stem rot can be obtained from the online Report on Plant Diseases about root and stem diseases of soybeans.
Results from previous field research on SDS showed that infection of the soybean root system occurred without the presence of observable foliar symptoms. In addition, we found that when severity of SDS was low to moderate, foliar symptoms were not always a good indication of yield potential. The implications of our results are that the fungus causing SDS colonizes the soybean root system and potentially causes reductions in yield even though the plant is showing no foliar symptoms. The effects of root infection are not as apparent as foliar symptoms and usually remain unnoticed unless the blue-colored spores of Fusarium solani f. sp. glycines, the causal organism, are observed on the roots of mature plants.
Signs of Fusarium solani f. sp. glycines on the soybean root system
This explains, in part, why this disease is difficult to control. There are still many unknown factors about this pathogen and how it is causing disease on soybeans. In addition, it is difficult to predict when, where, and how severe SDS will be because of the many environmental and other factors that influence disease development.
In the meantime, it is best to try to manage SDS by lessening its impact by way of the following steps:
1. Learn to identify SDS in the field, as symptoms may appear similar to more common diseases such as brown stem rot or stem canker.
2. Select soybean varieties that mature at different times. Use either different maturities within a maturity group or use different maturity groups. Early-maturing cultivars appear better.
3. Delay planting or extend planting time so that all soybeans are not at the same growth stage at the same time. However, do not wait past the suggested time for your area of the state.
4. Use cultural practices to improve drainage in low spots, reduce cyst nematode populations, and reduce soil compaction.
5. Crop rotation is of limited value since this organism can persist in the soil for many years. However, planting continuous soybeans is not recommended since this can increase other diseases.
6. The key to reducing yield losses from SDS is to plant cultivars with relatively high levels of tolerance or partial resistance to SDS. Information on SDS tolerance/resistance for commercial varieties from trials in Illinois can be found at the University of Illinois VIPS Web site and the Southern Illinois University SDS/SCN Web site.--Loretta Ortiz-Ribbing