Molecular Identification of Fungi Associated with Vascular Discoloration of Soybean in the North Central United States

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Abstract
Brown stem rot is a common but poorly understood vascular wilt disease of soybean. In order to more clearly delimit the causal agent (Phialophora gregata) and distinguish it from other morphologically similar fungi from discolored soybean stems, fungi were isolated on a semi-selective medium from discolored 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 longicolla was the most frequently isolated fungus, but it was isolated more commonly from lightly discolored or non-discolored stems than from discolored stems. 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 stem rot. In inoculation experiments, only P. gregata was capable of causing the vascular discoloration and leaf symptoms typical of brown stem rot; none of the seven isolates could be considered non-defoliating. Two other fungi, Plectosphaerella cucumerina and Gliocladium roseum, were similar in colony morphology to Phialophora gregata but were not pathogenic to soybean, and these may be the same species as those referred to by earlier workers as Acremonium spp. or the non-defoliating form of P. gregata.