Sudden death syndrome of soybean
Sudden death syndrome (SDS) is caused by the fungus Fusarium virguliforme. Foliar symptoms of SDS are a result of a toxin, produced by the fungus, moving from roots to the leaves. 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 petioles remain on the stem. Diseased plants have rotted taproots and lateral roots. When stems are cut lengthwise, the woody tissue of the taproot is discolored light gray to brown. 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 those of 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.
The fungus survives on infested crop residue or in soil for several years. Crop rotation with corn is not effective because the fungus can increase on corn residue. Root infection in soybean can occur within days of planting and is favored by high soil moisture. Significant rainfall during reproductive stages favors foliar symptom development. Soil compaction and infection by soybean cyst nematode (SCN) can increase disease severity.
Several soybean varieties have partial resistance (resistance made up of more than one gene) to SDS. Check with a local seed dealer to identify an appropriate variety. Effective seed treatments also can help reduce SDS. Reducing soil compaction and SCN population densities can decrease the risk of SDS.
Gallery Images: B. Kleinke, C. Bradley, M. Chilvers, D. Sjarpe, K. Wise, G. Munkvold, T. Hillyer, and D. Mueller