Variation Among Isolates of Phoma medicaginis var. medicaginis in Spore Production in Vitro and Symptom Expression on Excised Leaves of Alfalfa. Fred A. Gray, Professor of Plant Pathology, Department of Plant, Soil and Insect Sciences, University of Wyoming, Laramie 82071-3354. J. A. Fernandez, and J. L. Horton. Associate Professor of Plant Pathology, College of Agriculture, University of Hawaii at Hilo, Hilo 96720; and Research Associate, Department of Plant, Soil and Insect Sciences, University of Wyoming, Laramie 82071-3354. Plant Dis. 74:668-670. Accepted for publication 8 March 1990 . Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0668.

Excised trifoliolate leaves from a susceptible alfalfa (Medicago sativa) genotype were inoculated in vitro with 34 isolates of Phoma medicaginis var. medicaginis. Pathogenicity and virulence of isolates were assessed by their ability to produce leaf spot, petiole blight, and leaf chlorosis. Significant differences (P = 0.05) in the ability of isolates to produce symptom types were observed. There was a significant relationship between the ability to produce leaf spot and petiole blight (r = 0.33) but not between the ability to produce chlorosis and petiole blight (r = –0.12) or leaf spot (r = 0.22). Petioles were affected more than leaves. Density of spores produced on PDA varied among isolates (P = 0.05) from 3.5 × 10⁵ to 6.0 × 10⁸ / ml. However, no association between virulence and in vitro spore production or color of spore suspension was observed among the isolates.