By Doug Jardine
Within the past two weeks, SDS has been reported in Pawnee County, near Lawrence, and at the Kansas River Valley Paramore Unit. Wet soils typically trigger SDS development, so if August rains continue to fall, we can expect to see more symptom development, especially in fields with known histories of SDS.
SDS is a disease caused by the soilborne fungus Fusarium virguliforme. This fungus prefers wet conditions and thus is usually most severe in irrigated fields or dryland fields that receive significant amounts of rain during the early- to mid-reproductive stages. SDS tends to be most severe on well-managed soybeans with a high yield potential. It also tends to be more prevalent in fields that are:
- Infested with soybean cyst nematode
- Planted early when soils are cool and wet
- Compacted
- Historical yield losses from this disease are generally in the range of 1 to 25 percent.
Disease symptoms
Symptoms of SDS are easily recognizable. SDS begins as small, bright, pale green to yellow circular spots on the leaves during late vegetative or early reproductive growth stages. As the disease progresses, the tissue in these spots starts to die and enlarges to form brown streaks between the veins. Symptoms are more pronounced on top leaves. As the disease further develops, the leaflets drop off but the petioles remain attached.
Figure 1. Scattered yellow spots on some of the greener leaves in the lower right in this photo are the early leaf symptoms of SDS. The leaves in the center foreground have more advanced symptoms of SDS.
Figure 2. A soybean plant with leaf petals dropped but petioles remaining attached.
Flowers and pods may abort or not fill. Another key symptom of SDS is substantial amounts of root decay and discoloration of roots and crown.
Figure 3. Root rot occurs on plants infected with the SDS pathogen. This symptom distinguishes SDS from brown stem rot and stem canker.
Diseased plants are easily pulled out of the ground because the taproots and lateral roots have deteriorated. Symptoms present on both the leaves and roots are diagnostic for SDS. Positive diagnosis of the inner tap root is key to disease identification. Other problems such as triazole fungicide “burn,” and the diseases stem canker and brown rot, can give similar foliar symptoms. Please note, stem canker has also been reported in 2019 so be sure to check the tap roots when making a diagnosis.
Potential yield losses and management considerations
Soybean yield losses from SDS depend on both the variety and stage of crop development when the symptoms first appear. Appearance of the disorder at early pod fill is more damaging than its appearance at a later stage of plant development. Yield reduction is the result of reduced photosynthetic area, defoliation, flower and pod abortion, and reduced seed size.
Effective management of SDS requires an integrated approach. Management starts with the planting of SDS resistant varieties. Most varieties are susceptible to some degree and very few have excellent resistance. The most susceptible varieties yield 40 to 50 percent less than the resistant varieties at locations where SDS is present and yield levels are in the range of 60+ bushels per acre.
Figure 4. The variety on the right in a recent K-State performance test was susceptible to SDS. The foliage was completely dead by early pod fill.
Seed companies have SDS ratings for most of their varieties and there is typically a wide variation in ratings. There is little or no correlation between the maturity group of a variety and its SDS resistance rating.
The presence of SDS is strongly correlated with the presence of soybean cyst nematode (SCN). Therefore, where SDS is present, soil samples should be taken to determine the level of SCN present and it will need to be managed along with the SDS. However, producers cannot manage SDS simply by selecting varieties that have SCN resistance. Some varieties with good resistance to SCN are highly susceptible to SDS and some varieties that are susceptible to SCN are quite resistant to SDS. Ideally, producers should select varieties that are resistant to both SDS and multiple races of SCN.
In addition to resistant varieties, a second line of defense is the use of the planting time seed treatment ILeVO, which contains the active ingredient fluopyram. This product has performed well in several K-State research trials. Other seed treatments that have been evaluated were not as effective as ILeVO. The cost of ILeVO has come down since it was first introduced, but it is still costly insurance in fields not known to have SDS in them and we recommend its use only in fields with a history of the disease.
Cultural management practices that can reduce the risk of SDS infection include:
- planting SDS infested fields last when soil temperatures are warmer
- avoiding planting into overly wet soils
- reducing compaction problems within a field
Producers who have fields with compaction problems should make every effort to correct that problem before planting soybeans next season.
Crop rotation has a limited effect on SDS since the fungus has been shown to invade corn debris and survive saprophytically in the absence of a soybean crop.