With soybeans now in the mid- to late pod-filling stages, some stem and root rots are showing up. Brown stem rot and sudden death syndrome have been reported at several sites across Nebraska. With both of these diseases foliar symptoms can be similar so correct identification through other means is critical to successful management.
Brown stem rot has been present in Nebraska for many years. Some fields consistently have problems with this disease. Foliar symptoms are not always present, therefore any pockets of stressed or dying plants can be suspect for the disease.
Sudden death syndrome has been found in many Nebraska fields since it was first identified in 2004. In most affected fields the disease is contained to small areas, while most of the field remains unaffected. This year’s weather has not been very conducive to sudden death syndrome unless the field was irrigated. Soil compaction and high fertility levels also have been associated with increased levels of this disease.
Differentiating Brown Stem Rot and Sudden Death Syndrome
Brown stem rot can result in the same foliar symptoms as sudden death syndrome; however, damage to the stem can be used to differentiate the two diseases.
Foliar symptoms start with interveinal necrosis. The spots coalesce to form brown streaks between the leaf veins with yellow margins. The leaves eventually drop while the petiole (leaf stem) remains attached. The root system will have a deteriorated taproot and lateral roots will only be evident in the upper soil profile. Plants can typically be pulled from the ground very easily and there may be a dark blue fungal growth on the roots.
Plants affected by brown stem rot may have similar foliage symptoms, but will not have the root rot symptoms.
Split the Stems
With any root and stem rot disease it is critical that you split the soybean stem to properly identify the disease. With brown stem rot the center of the stem will be discolored and typically will be brown from the soil line going up (Figure 1). With sudden death syndrome discoloration will be confined to the outer layers of the split stem. The center will not be discolored, but the root cortex will be light gray to brown and may extend up the stem (Figure 2).
Management of Brown Stem Rot and Sudden Death Syndrome
The number one management tool for both of these diseases is variety selection so it’s important to keep records of affected fields and select the variety accordingly. Other management options vary with the disease. Fields with high levels of SDS should not be planted early in future years and should be managed to reduce any compactions issues. Fields with brown stem rot will benefit from longer rotations, but sudden death syndrome is not affected by rotation. Foliar fungicides are not recommended for managing either of these diseases.
As with any disease, correct diagnosis is critical to management. If you are uncertain of what’s causing the damage in your field, I encourage you to submit a sample to the Plant and Pest Diagnostic Clinic for identification. Foliar symptoms alone cannot be used to confirm any of the diseases discussed in this article as they can all look the same.