Molecular identification of fungi associated with vascular discoloration of soybean in the north central United States

Brown stem rot is a common but poorly understood vascular wilt disease of s oybean. In order to more clearly delimit the causal agent (Phialophora greg ata) and distinguish it from other morphologically similar fungi from disco lored soybean stems, fungi were isolated on a semi-selective medium from di scolored and non-discolored soybean stems collected at random across Iowa. Illinois, Minnesota, Missouri, and Ohio. A total of 11 fungi were commonly isolated and characterized based on colony morphology and DNA sequences of the internal transcribed spacer region of the rDNA operon. Phomopsis longic olla was the most frequently isolated fungus, but it was isolated more comm only from lightly discolored or non-discolored stems than from discolored s tems. Phialophora gregata was the next most frequently isolated fungus and was isolated more commonly from discolored stems and more commonly in 1996 than in 1995, which had a warm growing season and relatively little brown s tem rot. In inoculation experiments, only P. gregata was capable of causing the vascular discoloration and leaf symptoms typical of brown stem rot; no ne of the seven isolates could be considered non-defoliating. Two other fun gi, Plectosphaerella cucumerina and Gliocladium roseum, were similar in col ony morphology to Phialophora gregata but were not pathogenic to soybean, a nd these may be the same species as those referred to by earlier workers as Acremonium spp. or the non-defoliating form of P. gregata.