In late August 2002, patches of soybean (Glycine max) plants in a field in Richland County, ND exhibited symptoms and signs of charcoal rot and died prematurely. Dead plants had a silvery-gray appearance, and black microsclerotia (76 ± 28 μm in diameter) were present in the vascular tissue of the lower stems and roots. Stem tissue was placed in petri dishes containing potato dextrose agar (PDA). Gray hyphae grew from the stem tissue and subsequently turned black and formed microsclerotia (188 × 139 ± 48 μm). The fungus was identified as Macrophomina phaseolina (Tassi) Goid. based on colony color and morphology and size of the microsclerotia (1). To confirm pathogenicity, soybean plants (cv. Garst D 041 RR) were grown in the greenhouse and inoculated with the previously obtained isolate of M. phaseolina. Stems of soybean plants at the V2 stage were excised just below the third node. Mycelia plugs of a 1-week-old culture of M. phaseolina were placed into the large end of disposable micropipette tips (200 μl). The micropipette tips containing the M. phaseolina culture were subsequently placed over 10 excised soybean stems. To serve as a control, 10 excised soybean stems were inoculated with micropipette tips containing plugs of noninfested PDA. Thirty days after inoculation, micropipette tips were removed, and lesions that had developed on the stem were measured. The mean lesion length of M. phaseolina inoculated stems was 7 mm; no lesions developed on the control plants. M. phaseolina was reisolated from infected tissue of inoculated plants that were placed on PDA. Charcoal rot was only observed in the Richland County field; however, no surveys were conducted to determine the prevalence of the disease throughout the soybean production area of North Dakota. Because it is now known that this disease occurs in North Dakota, growers, extension personnel, and crop consultants must scout for the disease and practice recommended management strategies.
Reference: (1) G. S. Smith and T. D. Wyllie. Charcoal rot. Pages 29--31 in: Compendium of Soybean Diseases, 4th ed. G. L. Hartman, J. B. Sinclair, and J. C. Rupe, eds. The American Phytopathological Society, St. Paul, MN, 1999.
