Sudden death syndrome (SDS) of soybeans is a soil-borne fungal disease caused by Fusarium virguliforme.
This fungal infection infects the plant early in the growing season, however foliar symptoms of SDS typically do not appear until R3 (beginning pod) and later growth stages. Initial symptoms consist of chlorotic (yellow) spots on the leaves between the veins (Figure 76). As the disease continues to progress the yellow spots may coalesce, but stay between the veins at the leaf margin. Over time the yellow areas start to become brown (necrotic) in coloration as the tissue starts to die (Figure 77). The tissue death between the veins is commonly referred to as interveinal necrosis. As the disease continues to progress it is common to observe abortion of pods and plants and defoliation early in the growing season. The defoliated plants will retain their petioles (leaf stems) which is a symptom very commonly observed with SDS
With this disease, however, the fungus infects through the root system so closer examination of the root system is critical to identification. The roots will exhibit typical root rot commonly observed on the taproot. When the stem, crown, and root of the plant is split in half, discoloration of the vascular tissue is restricted to the outer stem area (xylem) and extends up into the stem from the soil line (Figure 78).The pith of the plant remains white in coloration (Figure 78). Figure79 shows the disease cycle SDS.
Sudden Death Syndrome Management
Varieties with resistance or tolerance to SDS should be considered in fields with a history of SDS issues
Avoid early planting fields with a history of SDS. Soil temperature below 60oF can favor SDS infection. Reducing compaction and improving soil drainage have been shown to reduce SDS risk.
This fungus can survive many years in the soil without a susceptible host thus crop rotation has not been shown to be an effective management tool.
Seed treatment fungicides are showing efficacy in managing early season infection of SDS.
Soybean Cyst Nematode (SCN) Control
SCN pressure can increase SDS levels because SCN weakens the plant and provides wounds for the fungus to invade and colonize root tissue.
Brown Stem Rot
Brown stem rot of soybeans is a soil-borne fungal disease caused by Phialophora gregata. The fungus invades through the roots in late spring with symptoms not developing until after pod formation. Foliar Commented [AT6]: https://www.youtube.com/watch?v=-KWu9dPCDXY symptoms observed are very similar to Sudden Death Syndrome with chlorotic (yellow) spots on the leaves between the veins. As the disease continues to progress the yellow spots may coalesce, but stay between the veins at the leaf margin. Over time the yellow areas start to become brown (necrotic) in coloration as the tissue starts to die (Figure 80).The tissue death between the veins is commonly referred to as interveinal necrosis. The leaves eventually wilt and die.
Examination of the stem is critical in identification of this disease. Splitting of the crown and soybean stem will expose a brown to reddish brown discoloration of the stem pith. With severe infection the pith may have the appearance of corrugated cardboard (Figure 81)and can continue throughout the stem of the plant.
Brown Stem Rot Management
Planting non-host plants such as corn or small grains will prevent build-up of the brown stem rot fungus.
There is limited sources of genetic resistance available but when used in combination of other management strategies, it is effective management tool.
Planting soybean varieties with shorter relative maturity and avoid planting in narrow rows. Also maintaining optimum soil fertility and pH for soybean production.
Research has shown that tillage reduces the amount of brown stem rot. A study completed in Wisconsin showed that brown stem rot was 30% greater and yields were 15% lower in no-till than in conventional tillage fields.