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First Report of Northern Stem Canker of Soybean Caused by Diaporthe phaseolorum var. caulivora in North Dakota

In early September 2003, patches of soybean (Glycine max) plants in a field in Foster County, ND had dead branches with reddish brown cankers at the nodes. Stem tissue with cankers from two plants were excised and immersed in a 0.5% NaOCl solution for 1 min, rinsed with sterilized distilled water, and placed into Petri dishes containing potato dextrose agar (PDA) amended with streptomycin sulfate (200 mg/liter). Four fungi were hyphal tipped and grown on PDA, acidified PDA, or water agar (WA) with soybean stem pieces incubated at 21°C with 24-h light and identified by culture and spore morphology after 3 to 4 weeks. Cultures were identified as Diaporthe phaseolorum var. caulivora that produced white colonies with interspersed cottony tufts of mycelium on PDA (1). Stromata were small and scattered irregulary on the plate. Perithecia of the D. phaseolorum var. caulivora isolates were black and globose with eight-spored asci formed on soybean stem pieces on WA. To confirm pathogenicity of the D. phaseolorum var. caulivora isolates, soybean plants (cv. NuTech 0606) were grown in the greenhouse and inoculated with recovered D. phaseolorum var. caulivora isolates. The stems of soybean plants at the V3 stage were excised just below the fourth node. Mycelia plugs of the four different 1-week-old cultures of D. phaseolorum var. caulivora were placed into the large end of disposable micropipette tips (200 μl). The micropipette tip containing a D. phaseolorum var. caulivora culture was subsequently placed over a cut soybean stem. The four D. phaseolorum var. caulivora isolates were used to inoculate 10 stems apiece. To serve as controls, 10 cut soybean stems were inoculated with micropipette tips containing plugs of noninfested PDA and 10 cut stems were not inoculated. Ten days after inoculation, micropipette tips were removed and lesions that had developed on the stem were measured. The mean lesion lengths of the D. phaseolorum var. caulivora inoculated stems ranged from 48 to 82 mm and were significantly (P ≤ 0.05) greater than the lesion lengths of the noninfested PDA (3 mm) and no PDA (1 mm) controls. Northern stem canker was only observed in the one field; however, surveys were not conducted to determine the prevalence of the disease throughout the soybean-production area of North Dakota. To our knowledge, this is the first report of northern stem canker on soybean in North Dakota.

Reference: (1) F. A. Fernandez et al. Stem canker. Pages 33--35 in: Compendium of Soybean Diseases. 4th ed. The American Phytopathological Society, St. Paul, MN, 1999.