A soybean field with moderate level of white mold.
Common Late-Season Soybean Diseases
Late-season soybean diseases can sometimes be mistaken for natural senescence. A closer look at the stems and roots of dying plants and the pattern displayed by dead plants in the field may reveal root or stem rots going on. In order to devise effective management practices for future soybean seasons, it is important to determine the cause of early soybean plants death.
The common-late season root and stem rot diseases of soybeans are Sudden death syndrome (SDS), Phytophthora root and stem rot, stem canker, white mold, brown stem rot, and charcoal rot. Infections by the pathogens that cause these diseases happen early in the growing season, but symptoms develop later and can be exacerbated by environmental stresses.
Sudden Death Syndrome
Sudden death syndrome (SDS), while not very common in South Dakota, has been found in a few fields this season. The SDS pathogen infects seedlings early in the season but symptoms do not develop until after the flowering growth stage. SDS symptoms start as small chlorotic spots on leaves. These quickly expand leading to large yellow blotches that eventually become necrotic (dead leaf tissue) between leaf veins, leaving green veins (Figure 1 inset). Infected plants have rotted roots. Splitting the stem lengthwise reveals the browned cortex and white pith.
Figure 1. Sudden death syndrome symptoms. Inset: a close-up of SDS symptoms on the leaf.
Phytophthora Root & Stem Rot
Phytophthora root and stem rot can develop in soybeans anytime during the growing season. However, the most important damage caused by Phytophthora root and stem rot is when soybean seedlings and young plants (before flowering) are killed. Late Phytophthora rot and stem rot can also occur, especially when plants are under stress. Field entry points are the most-impacted areas of the disease (Figure 2), mainly due to compaction and low spot areas. These would be areas that typically have poor drainage.
Plants infected with Phytophthora root and stem rot pathogen have rotted roots and a brown lesion extending up the stem starting just above the soil line. Delayed development of Phytophthora root and stem rot can be due to tolerance of the soybean cultivar planted.
Figure 2. Soybean plants with Phytophthora root and stem rot near a field entrance.
Stem canker is also a late-season soybean stem rot disease, and it can kill plants in large portions of a field. Early symptoms of stem canker include dead petioles attached to branches (Figure 3). This can happen without necessarily causing the canker on the stem. The canker starts as a small brown lesion at the point of the petiole attachment to the branch and girdles the stem, expanding in both directions, causing wilting, and eventually killing the plant (Figure 2 inset).
Figure 3. A soybean field with high incidence of stem canker. Notice the dead petioles. Inset: Extended stem canker lesion that has girdled the stem.
White mold develops from mid-season to late season (Figure 4). 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 (Figure 4 inset). This disease will develop under high-moisture and warm-weather conditions. Narrow row spacing, high fertility (especially from animal manure), and previous occurrence of white mold are the risk factors for white mold to develop.
Figure 4. A soybean field with moderate level of white mold. Notice the cotton-white mycelia and black sclerotia (the survival structures of the fungus) on the inset picture.
Brown Stem Rot
Brown stem rot is another disease that develops in soybean mid-to-late season. Plants infected by the brown stem rot pathogen show no foliar symptoms until early pod set. Leaf symptoms on diseased plants sometimes may not develop, depending on the environment. When leaf symptoms develop, leaves have yellow-brown blotches between veins (Figure 5). The best way to confirm brown stem rot is to split the soybean stem longitudinally. The presence of a dark brown discoloration of the vascular tissues is an identifying characteristic of brown stem rot (Figure 5 inset).
Figure 5. Brown stem rot symptoms on soybean leaves. Inset: A soybean stem split to reveal the browned pith, a typical symptom of brown stem rot.
Charcoal rot usually develops under moisture stress conditions. The charcoal rot pathogen clogs roots leading to blockage of water and mineral salts. Plants affected by this disease may be in patches or scattered in the field (Figure 6). Infection takes place early in the season but symptoms develop later on. Charcoal rot can be confirmed by splitting the soybean stem or peeling the taproot skin and observing small embedded black spots that resemble granules of charcoal (Figure 6 inset).
Figure 6. Soybean plants prematurely killed by charcoal rot. Inset: A split root showing charcoal rot microslerotia, a sign of charcoal rot pathogen.
Root & Stem Rot Management
Root and stem rots that develop during the growing season have no in-field treatments. However, determining the diseases affecting your soybeans can help with management decisions for future seasons. The pathogens that cause these diseases survive on the soybean residue and in soil. Therefore practices such as crop rotation, tillage to manage residue help to reduce level of inoculum.
Select a soybean cultivar that has resistance or tolerance to the diseases observed on your farm. Resistant cultivars are available for Phytophthora root and stem rot management. For other diseases, tolerant cultivars may be available. Check the seed catalogue for ratings for different diseases. Improving drainage, planting appropriate plant populations, and good soil fertility program will reduce plant stress and limit some of these diseases from developing.