Both Diaporthe phaseolorum var. caulivora and D.phaseolorum var. meridionalis cause stem canker on soybean, with D. phaseolorum var. caulivora reported in the northern regions and D. phaseolorum var. meridionalis reported in the southern regions of the United States (1). During the 1999 and 2000 growing seasons, fungi were isolated from soybean plants from growers' fields exhibiting stem canker symptoms. Stem tissue along the margin of the canker was cut into 1- to 5-mm3 pieces, surface-disinfected for 4 min in 0.5% NaOCl solution, rinsed twice, and plated on water or potato dextrose agar (PDA). Fungi of interest were hyphal tipped, grown on PDA at 21°C with 24 h of light, and identified by culture and spore morphology after 3 to 4 weeks. Typical D. phaseolorum var. meridionalis isolates produced white, lanose colonies that turned tan with age. Most of the D. phaseolorum var. meridionalis isolates produced pycnidia with alpha spores and beaked perithecia after 25 to 30 days (2). Brown to black stromata formed in irregular shapes. Of the 16 D. phaseolorum var. meridionalis isolates identified, 11 were from Illinois, 1 each from Indiana and Ohio, and 3 from Kentucky. In Illinois, four isolates were from the northern part of the state, and the rest were from the central and southern areas of the state. In addition to D. phaseolorum var. meridionalis, other isolates obtained from soybean plants included D. phaseolorum var. caulivora, D. phaseolorum var. sojae, and Phomopsis longicolla.
References: (1) J. A. McGee and D. C. Biddle. Plant Dis. 71:620, 1987. (2) A. W. Zhang et al. Phytopathology 88:1306, 1998.