First discovered in Arkansas in 1971, Sudden Death Syndrome is now know as one of the most important diseases of soybeans in the Midwest. This fungal disease is most severe when soybeans are planted early in the spring into cool and wet soils followed by heavy midsummer rains. SDS is caused by a soilborne fungus fusarium solani f. sp gylcines that overwinters in crop residue or freely in the soil and can also survive in cysts of the soybean cyst nematodes. As the soil temperature rises in the spring, the soybean roots are stimulated to germinate and effect the roots. The fungus may effect the roots as early as a week after emergence but the symptoms will not be known until mid-July- August when the reproduction stage is happening.
Symptoms
The first noticeable symptom of SDS is yellowing and defoliation of the upper leaves of the soybean plant. This will first appear in small areas or strips in the field and slowly enlarge to other areas of the field. Spots where wetter and compacted soils such as field driveways or turn rows, SDS will be most noticeable. The overwintering of the fungus allows the symptoms to grow larger and affect more of the field from year to year. As the symptoms develop mottling and mosaic will take place in the leaves as seen is Figures 1 & 2. Then the leaf tissue between the major veins will turn yellow to brown where they will die off. Examining the interior of the lower stem and taproot will provide a more accurate diagnostic. When you split the lower steam and taproot it will exhibit a slightly tan to brown discoloration compared to a healthy plant as seen in Figures 3 & 4. This fungus will develop toxins that will only invade the soybean plant above the soil line a few inches but does not invade the leaves, flowers, pods, or seeds.
Figure 1: Note the development of the mosaic leaves as the SDS develops. Figure 2: Mosaic Soybean Leaf
Figure 3: Split stem: SDS on the left, healthy on the right. Figure 4: SDS infected roots
Management Practices
While management for Sudden Death Syndrome is limited there are a few options to help reduce the risk of SDS damage.
Planting Date: When we plant our soybeans early in the spring we predispose the soybean to the fungus expecially when the soils is wet and cool. If early spring conditions are favorable for rapid soybean growth and if saturating rains do not occur during reproductive stages the risk of SDS is limited even though the fugus is still in the soil. Growers should consider planting fields with a history of no SDS first hen moving to the problem fields later in the planting season.
Tillage: Compacted and wetter soils restrict the development of the root system. When we receive a heavy rain during the reproductive stage of the soybean, SDS starts to develop. By correcting the soil compaction and ultimately allowing for water permeability will help reduce the risk of SDS. Therefore, producers should create the best drainage system for their soils weather it is intensive tilling or with other soil types it may be a no- till operation.
Resistant Soybean Varieties: Breeders are working on developing a SDS resistant variety but it has been a slow process. Most seed companies have removed highly susceptible varieties from their line ups but there is still no highly resistant variety for SDS. It is very critical that producers receive accurate information for new varieties to help insure low susceptible varieties with any new products.
Rotation: Research has found that there does not seem to be a significant to reducing SDS with crop rotation. With a rotation of corn and soybeans, it seems to be to short to reduce the population and infection of SDS. Even with continuous corn for many years coming back with a soybean crop in a field known for SDS can still be servere with the right conditions.
When dealing with SDS in your fields be sure to keep accurate records of when, where, and the severity of this fungus. Fields that are known to be infected with SDS should be one of the later fields planted in the spring, consider tillage practices, and varieties with some type of resistance.
