Sudden death syndrome (SDS) is becoming increasingly prevalent throughout Iowa. Yield losses may range from a few percent to almost 100 percent. Foliar symptoms rarely appear until after flowering. Leaves of infected plants initially show scattered, yellow spots between leaf veins. Spots grow to form large chlorotic and necrotic blotches between the leaf veins, while the midvein and major lateral veins remain green. Leaflets eventually drop, but the petioles remain on the stem.
Diseased plants have rotted taproots and lateral roots. When stems are cut longitudinally, the woody tissue of the taproot is discolored light gray to brown (see page 23). This discoloration may extend up to two inches above ground. Bluish fungal growth may be seen on the surface of roots if soil moisture is high. Foliar symptoms of SDS look similar to brown stem rot (BSR). A good way to distinguish between BSR and SDS is the presence or absence of internal stem browning. Stems of SDS-infected plants have white pith, while BSR causes brown discoloration of the pith. Another way to distinguish between the two is to check roots of diseased plants. Plants with SDS have root rot, while BSR does not cause root rot.
The fungus survives on infested crop residue or in soil for several years. Root infection can occur within days of planting and is favored by high soil moisture and cool temperatures. Significant rainfall at or near the flowering stage favors foliar symptom development. Foliar symptoms are a result of a toxin produced by the fungus moving from roots to the leaves. Soil compaction and the presence of soybean cyst nematode can increase disease severity.
Best time to scout is R1 through R6, check plants in areas with high soil moisture and in fields infested with soybean cyst nematode. Split the stem to distinguish from brown stem rot.
Variety selection: Several soybean varieties have partial resistance to SDS. Check with a local seed dealer to identify an appropriate variety.
Reduce soil compaction: Increased SDS severity has been associated with compacted soil. Deep tillage may reduce severity of SDS where the soil is compacted. Tillage may also promote earlier warming of soils. If tillage is considered to reduce soil compaction, great care should be taken to minimize soil erosion and maintain soil quality.
Planting date: The fungus prefers cool soil for infection. Delaying planting by a week or two may reduce the risk of disease due to planting in warmer soils.
Reduce soybean cyst nematode (SCN) populations: Populations of SCN are usually, but not always, associated with SDS and may increase its severity, especially in varieties that are SCN susceptible. Management practices to reduce SCN population densities, including variety selection and preventing the spread of soil from field to field, may delay onset and spread of SDS.
Crop rotation: Crop rotation with corn is not effective. Residue such as corn kernels and corn roots harbor the SDS pathogen in the soil. SDS outbreaks can occur even after a few years of continuous corn.
Photo by Brandon Kleinke and Daren Mueller
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