Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
We have observed stem canker at low levels in soybean fields throughout Delaware. Stem canker is caused by fungal pathogens that infect and girdle the stems, restricting water and nutrient movement throughout the plant. This action results in premature death or poor grain fill of affected plants. Symptoms first start as red/brown lesions at the nodes of plants near the reproductive phase of growth. These lesions are surrounded by green tissues above and below the lesions (Figure 1).
Over time, the lesions expand, turn blackish in color, and become sunken. These sunken lesions are called cankers, hence the clever, original, and highly innovative disease name. Several cankers may be present on a plant, and may grow together to form large areas of affected stem tissues. As these lesions age clusters of black fruiting bodies are found within the lesions, which, if viewed from the side with a hand lens, project outward from the stem (Fig 2)
Figure 1. Examples of stem canker symptoms on soybean stems. Note the presence of green tissue above and below the lesions. Image obtained from Wise et al. 2015. Scouting for Soybean Stem Diseases. Crop Protection Network.
Figure 2. Stem canker lesions may contain patches of black fungal fruiting bodies with long “beaks.” These structures can produce spores that can cause secondary infections of plant nodes on the plant or surrounding plants. Secondary spread of the pathogen has not been shown to affect disease development or impact yield.
In addition to symptoms on the stems, affected plants will present foliar symptoms, due to restriction of water to the canopy as well as the movement of a fungal toxin, produced by the pathogen, which accumulates in the foliage. Toxin accumulation results in leaves with interveinal chlorosis, meaning that the veins remain green and the tissue between the veins turns yellow to brown (Figure 3). This symptom is shared with other soybean diseases you may encounter such as Sudden Death Syndrome (SDS). The stem discoloration distinguishes this symptom from SDS or other issues such as Soybean Vein Necrosis Virus, compaction, or nutrient deficiencies. Shepard’s cooking, or bending, of the upper stems may also occur.
Figure 3. Foliar symptoms of stem canker are similar to SDS and other issues in soybeans. The presence of lesions on the stems help distinguish stem canker from other diseases such as SDS.
Severe infections from stem canker occur when the plants are infected during the vegetative growth (V1-V7). Infection is favored during extended periods (1-4 days) of moderate temperatures (72-86°F) and wet weather. Severely affected seedlings infected in the early vegetative stages may die rapidly. However, many times symptoms may not be evident until the plant reaches the reproductive phase due to the fungus existing asymptomatically within tissues prior to causing symptoms. This is also known as the latent period of the pathogen. Once symptoms are noted, within season management with fungicides will not save affected plants as the fungus is already established within tissues. In addition, secondary spread is not likely to impact yields, as this occurs later in the season, and dispersion of spores from fruiting bodies is limited to 3-6 feet from the source. After the growing season the fungus persists in soybean residues, which serve as sources of the pathogen in the subsequent growing season.
Management of stem canker is achieved through implementing several cultural practices. First, rotate your soybeans with other non-host crops for 1-2 years, such as corn, small grains, or vegetables. Second, promote residue decomposition in affected fields if possible. Third, plant high quality, certified seed of varieties with resistance to stem canker. Fourth, avoid planting soybeans excessively early as this may coincide with weather conditions that favor severe infections of soybeans at the vegetative stages of development. Double cropped soybeans are unlikely to suffer from significant stem canker issues in most instances. Fungicide sprays targeted at the early vegetative stages are most efficacious. However, they are unlikely to be needed if the aforementioned cultural practices are being followed. Fields containing susceptible varieties planted into soybean residue that have had stem canker issues confirmed by a reputable diagnostic clinic in the recent past, are most likely to have issues with stem canker in a particular growing season.
More information on stem canker on soybean can be found at the NCSRP website at: http://soybeanresearchinfo.com/pdf_docs/StemCanker_CPN1006.pdf