Occurrence of Soybean Stem Canker (Diaporthe phaseolorum var. meridionalis) in Wisconsin

Soybean (Glycine max) developed symptoms characteristic of stem canker during the 2000 to 2003 growing seasons in Wisconsin. Symptoms were widespread in 2003 and were associated with yield losses of ≈1% statewide and as much as 25% in individual fields. Affected plants expressed dieback of foliage beginning at growth stage R3 and progressed until the R6 growth stage. Dark brown lesions were frequently observed at a single node on the lower portion of stems of plants expressing foliage dieback. Fungi were isolated from symptomatic plants collected from seven growers' fields in Rock, Sauk, Veron, and Walworth counties and the Arlington and Marshfield Agricultural Research Stations. Stems with lesions were cut into ≈5-mm pieces, surface disinfested with a 0.5% NaOCl solution for 3 min, rinsed three times in sterile distilled water, and placed on water agar (WA) or potato dextrose agar (PDA) at pH 4.5. Hy-phal tips from colonies of interest were excised and placed on acidified PDA at 25°C under continuous light for 25 to 30 days. In addition to Diaporthe phaseolorum var. caulivora (the cause of northern stem canker), four isolates of D. phaseolorum var. meridionalis (the cause of southern stem canker) were isolated. Colonies of D. phaseolorum var. meridionalis isolates were white, lanose, and became tan with age as previously described for D. phaseolorum var. meridionalis (1). Pycnidia with alpha conidia (no beta conidia) and perithecia with 3.1 to 3.4 × 9.5 to 9.8 μm ascospores formed on oat flakes on acidified WA after 30 days. Stromata were brown to black and irregularly shaped. Four isolates of D. phaseolorum var. meridionalis were tested for pathogenicity in a controlled environment using a cut stem inoculation method (2). Stems of 3-week-old seedlings of cv. Sturdy were cut at the midpoint between the second and third node, and a PDA mycelial plug (4 mm in diameter) was placed on the surface of the cut stems. This method was used to inoculate 15 plants in three replicates for each isolate tested. Inoculated plants were placed in a mist chamber in the dark at 25°C for 4 days and later moved to a greenhouse with a 16-h photoperiod at 24 ± 3°C for 3 days. All plants challenged by this method exhibited stem lesions that were 2 to 3 cm long and of similar color to lesions observed in field-grown plants. For each isolate tested, D. phaseolorum var. meridionalis was reisolated from three randomly selected symptomatic plants. Negative controls with a PDA plug did not produce lesions. To our knowledge, this is the first report of D. phaseolorum var. meridionalis on soybean in Wisconsin. The significance of this report relates to the potential spread of D. phaseolorum var. meridionalis beyond its known southern range in the United States. References: (1) F. A. Fernandez et al. Stem Canker. Pages 33-35 in: Compendium of Soybean Diseases, 4th ed. G. L. Hartman et al., eds. The American Phytopathological Society, St. Paul, MN, 1999. (2) S. Li et al. Plant Dis. 85:1031, 2001.