By Doug Jardine
The first sudden death syndrome (SDS) infected soybean plants arrived in the K-State Plant Disease Diagnostic Lab the week of August 21st. The two samples originated in Cherokee and Johnson Counties. In areas that have a history of SDS, recent rains or irrigation can stimulate the development of the disease, so scouting should be going on now.
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 field in Franklin County with SDS.
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.
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 is significant so it is recommended for use only in fields with an established history of SDS. Prophylactic use of the product as “insurance” is not cost effective.
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 also seems to have some positive effect on SDS, but only if the field is not planted to soybeans for four years or more.