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What is Killing Your Soybean Plants?

Several soybean fields scouted in a number of counties have plants dying from various diseases. Most common diseases were sudden death syndrome (SDS) in three counties (Kingsburry, Beadle, and Davison), Phytophthora root rot (all counties scouted), brown stem rot (Davison County), northern stem canker (Davison and Turner Counties), and white mold (Brookings County).

The sudden death syndrome pathogen infects plants early in the season under cool and wet soil conditions but foliar symptoms do not develop until after flowering. Rainfall around flowering time increases the chances of SDS foliar symptoms development. Foliar symptoms start with small pale green circular spots that expand to become flashy yellow irregular interveinal blotches. The blotches eventually turn brown and may lead to premature leaf fall. SDS can be diagnosed by splitting the soybean stem and examining the taproot. Infected plants have discolored taproot with gray-brown cortex.

Like SDS, the Phytophthora root rot (PRR) pathogen infects plants at an early stage when soils are excessively wet but can continue to kill plants throughout the growing season. Plants being killed by PRR at this time are stressed plants due to moisture stress. PRR is diagnosed by the extended brown lesion above the soil line.

Brown stem rot also infects young plants through the roots but symptoms are expressed under cool wet weather during pod-fill growth stage followed by hot dry weather. Symptoms are best seen at R5 soybean growth stage. Brown stem rot can be distinguished from SDS by splitting the stem. Extensive browning of the vascular elements and the pith (Fig. 4) is the diagnostic sign of brown stem rot pathogen.

Northern stem canker can develop on both leaves and stems. It starts with small reddish brown lesions usually on lower nodes. These expand longitudinally girdling the stem leading to wilting and eventually dying of the plant. Plants killed by northern stem canker can expand to large portions of the field.

White mold is a high humidity disease that develops after the flowering growth stage. White mold can easily be diagnosed by the cottony-white mycelia on the stem and by the black hardened fungal mass (sclerotinia) on and/or within the stem. This disease will develop under high moisture and hot weather conditions especially in drilled or narrow row spacing.

Hail injury was observed in Davison County and severely injured plants may display symptoms that may resemble other late season diseases. The wounds act as entry points for fungal and bacterial pathogens especially Diaporthe spp.

Source: Emmanuel Byamukama, iGrow

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