UNL CropWatch August 4, 2010: Stem and Root Rots in Soybeans -- Identification and Management

UNL CropWatch August 4, 2010: Stem and Root Rots in Soybeans -- Identification and Management

August 4, 2010

As soybeans are in the mid-pod filling stages, we are starting to see stem and root rot diseases. So far we have received reports of sclerotinia stem rot and sudden death syndrome.

Photo - Sclerotinia soybean stem damage

Photo - Sclerotinia plant damage

Sclerotinia symptoms in soybean (Clockwise from top): Dark fungal bodies (sclerotia) in stems; white growth on stems with stems appearing bleached on dying plants.
Photo - SDS damaged leaf Sudden death syndrome leaf symptoms start as chlorotic blotches between the leaf veins that turn into brown dots that coalesce into brown streaks with yellow margins betwen leaf veins.
Photo - Brown stem rot damage Brown stem rot symptoms: (left) Typically the stem will be discolored from the soil line up. Symptoms are similar to those of SDS so it's important that you split the stem to identify stem damage. With SDS the interior plant stem is not discolored.

Sclerotinia Stem Rot

Sclerotinia stem rot (white mold) is not common in Nebraska soybeans. It’s been about 10 years since the last widespread outbreak in Nebraska. This year’s cooler temperatures and wet conditions during flowering were favorable for disease development in some areas of the state. This disease usually appears as isolated pockets of dying soybean plants.

Disease symptoms include a white cottony mycelium (fungal growth) growing on the stems, which will appear bleached. Foliar symptoms are similar to those of sudden death syndrome or brown stem rot. As long as the hot weather continues we should not see significant spread of this disease. Currently there is no spore spread and further spread of the disease is limited by growth of the fungus from the affected plants to adjacent plants. At one time or another many Nebraska fields have had Sclerotinia and the fungus is just waiting for good environmental conditions to start development.

Sudden Death Syndrome

Sudden death syndrome (SDS) has been found in many Nebraska fields since it was first identified in 2004. Most affected fields have small areas with the disease and large areas that are not impacted. This year’s weather pattern, with both early season moisture (after planting) and moisture at the early reproductive stages, has been conducive to SDS. Also contributing to increased levels of SDS are earlier planting dates, soil compaction, and high fertility levels.

The foliar symptoms start with interveinal necrosis. The spots coalesce to form brown streaks with yellow margins between the leaf veins. Eventually, most leaves drop with the petiole (leaf stem) remaining attached. The root system will have a deteriorated tap-root and lateral roots will only be evident in the upper soil profile. Typically, plants will be easily pulled and there may be a dark blue fungal growth on the roots.

Disease Identification

With any root and stem rot disease it is critical that the stems are split to properly identify the disease. Brown stem rot is common in Nebraska and will have the same foliar symptoms as SDS. To identify the disease, split the stem and examine damage.

  • With SDS the center of the plant stem will not be discolored; discoloration will be confined to the outer stem layers. The root cortex also will be discolored and will be light-gray to brown; this discoloration may extend up the stem.
     
  • With brown stem rot the center of the stem will be discolored and typically be brown from the soil line up.
     
  • Plants with sclerotinia will usually have dark fungal bodies (sclerotia) inside. The outside of the stem will appear bleached.

Disease Management 

Management for stem and root rot varies with the disease. It is important to keep records on affected fields and use them to inform future variety selection, the number one management tool available. Foliar fungicides are not recommended for managing any of these diseases.

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; however, SDS and Sclerotinia are not affected by rotation.

As with any disease, correct diagnosis is critical to proper management. If you are uncertain of the cause of damage in your field, I encourage you to submit a sample to the UNL 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.

More information on these and other soybean diseases can be found at Plant Disease Central and in the Soybean section of CropWatch under Disease Management.

Loren Giesler
Extension Plant Pathologist


 

Online Master of Science in Agronomy

With a focus on industry applications and research, the online program is designed with maximum flexibility for today's working professionals.

A field of corn.