The 2014 season may be, as Yogi Berra said, déjà vu all over again. Although southern soybean stem canker has been a rare occurrence in Arkansas for the past several years, the disease is still around. Last week a few plants with stem canker were found in a research plot on the Rice Research and Extension Center, reminding us that southern stem canker has not entirely vanished from Arkansas soybean fields. Wet, cool springs, particularly with frequent rain events that occur on plants during the vegetative stages (V3-V10), are ideal for the development of stem canker because infection occurs when spores are splashed onto young leaves. The fungal pathogen, Diaporthe phaseolorum var. meridionalis infects the leaflet and grows down the petiole to the main stem where it may remain more or less latent until the plant enters the reproductive phase of development. Leaf symptoms are usually the first to be noticed (Figure 1), but the presence of a canker that begins at a leaf node and extends up one side of the stem (Figure 2) is the primary evidence that the plants have stem canker and not sudden death syndrome or some other malady.
Fortunately, most of the soybean cultivars that have been screened over the last 10 years through the Arkansas Soybean Cultivar Disease Screening Program, supported annually by the Arkansas Soybean Promotion Board (http://arkansasvarietytesting.com/home/soybean/), are resistant to southern stem canker. A few moderately susceptible and susceptible cultivars come on the market each year, however. The next few weeks will be an excellent time to scout fields in which susceptible cultivars are growing for the presence of this disease. There is no remediation for the disease this year if it is present in a field, but knowledge is power, and identifying fields where the disease is present will make cultivar selection much more effective for next year. If stem canker is suspected, the Plant Disease Clinic can provide accurate diagnosis. For more information on southern stem canker see the Arkansas Soybean Production Handbook at http://www.arkansas-crops.com/wp-content/uploads/2014/02/mp197.pdf.