First Report of Charcoal Rot (Macrophomina phaseolina) on Soybean in Minnesota.

In August 1999, soybean (Glycine max (L.) Merr.) plants exhibiting symptoms of charcoal rot were observed near Zumbrota, MN. Symptoms included shrunken, unfilled pods, and brown, wilted leaves attached to dead petioles and stems (1). When stems of symptomatic soybean plants were split, areas of gray-to-black discoloration where present in the stem cortex (1). Black, spherical microsclerotia 77 to 90 µm in diameter and elongated microsclerotia 77 to 120 µm long (1) were found in vascular tissue. Stem tissue placed on potato dextrose agar (PDA) yielded fungal colonies identified as Macrophomina phaseolina (Tassi) Goid. based on gray colony color, colony morphology, and the presence of microsclerotia 70 to 90 µm in diameter. In 2000, M. phaseolina was isolated from plant samples gathered from 11 of 90 fields sampled in a statewide soybean disease survey. More studies are needed to determine the distribution of charcoal rot in Minnesota; however, the occurrence of symptoms at one location and the presence of M. phaseolina in soybean-growing areas of Minnesota suggest that charcoal rot may occur in susceptible soybean cultivars under favorable environmental conditions. Reference: (1) G. S. Smith and T. D. Wyllie. Charcoal rot. Pages 29-30 in: Compendium of Soybean Diseases, 4th ed. G. L. Hartmann, J. B. Sinclair, and J. C. Rupe, eds. The American Phytopathological Society, St. Paul, MN, 1999.