August 8, 2013

Stem and root rots are showing up in soybean a little earlier this year than in previous years. Over the past couple weeks we’ve seen some brown stem rot and, with the recent heavy rains, some Phytophthora. Typically, we do not see brown stem rot or sudden death syndrome until the mid to late pod-filling stages. We do not expect to see much SDS this year as conditions were dry during harvest (less compaction) and many areas were planted later due to spring rains.  (Also see August 22 update to this report with information on Sclerotinia Stem Rot.)

Figure 1. Foliar symptoms of brown stem rot of soybean.

Soybean Disease Identification

Brown stem rot has been in Nebraska for many years and some fields will consistently have problems with this disease. It can be suspected in any pockets of stressed or dying plants. Foliar symptoms are not always present, but when present will start with interveinal necrosis and spots that coalesce to form brown streaks between the leaf veins with yellow margins. Plants affected by brown stem rot will have some root rot, but it won’t be as extensive as in plants with SDS.

To differentiate brown stem rot from SDS, note these SDS symptoms

• the root system will have a deteriorated taproot;
• lateral roots will only be evident in the upper soil profile;
• plants will typically pull very easily; and
• there may be a dark blue fungal growth on the roots.

Plants affected by brown stem rot will not have the root rot symptoms.

Figure 2. Split soybean stem showing internal stem discoloration at the nodes due to brown stem rot of soybean.

Figure 3. Phytophthora of soybean. Note the dark discoloration of the stem at the base of the plant and extending up the stem.

Split the Stems. With any root and stem rot disease it is critical that the stems be split to properly identify the disease. Brown stem rot will have the same foliar symptoms as SDS but different stem symptoms.

With brown stem rot the center of the stem will be discolored while with SDS, the outer stem layers will be discolored. With brown stem rot the center of the center stem discoloration will typically be brown from the soil line upward. In some cases discoloration will start at the nodes and may be visible when the stems are split. With SDS the root cortex will also be discolored and will be light-gray to brown and may extend up the stem.

Fields affected by Phytophthora will have plants wilting and dying but there will be a discolored stem base on the plant (evident on the outside). The darker stem color will extend from the soil line and be above the first node in most cases. Heavy rains and saturated soil conditions favor disease development. Over the past several years we have observed that fields with light drought stress followed by heavy rains tend to develop more Phytophthora.

Management

Management for stem and root rot varies with the disease. It is important to keep records on affected fields so that variety selection can be used to manage future outbreaks. Variety selection is the number one management tool for all these diseases.

In future years fields with high levels of SDS should not be planted early and should be managed to reduce any compaction. Fields with brown stem rot will benefit from longer rotations. SDS will not be affected by rotation. Phytophthora can be managed with resistant varieties and seed treatments.

Foliar fungicides are not recommended for managing any of these diseases.

Professional Diagnosis

Correct diagnosis is critical to managing any soybean disease. If you are uncertain of the cause of damage in your field, I encourage you to have it identified at the UNL Plant and Pest Diagnostic Clinic. Foliar symptoms alone cannot be used to confirm any of the diseases discussed in this article as they can all look the same.

More information on these and other soybean diseases can be found on two UNL websites: PDC.unl.edu and CropWatch.unl.edu.

Loren Giesler
Extension Plant Pathologist