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Pathogenicity of Phoma medicaginis var. medicaginis to Crowns of Alfalfa. R. Rodriguez, Department of Plant Pathology, The Pennsylvania State University. K. T. Leath, U.S. Department of Agriculture Regional Pasture Research Laboratory, University Park, PA 16802. Plant Dis. 76:1237-1240. Accepted for publication 7 August 1992. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1992. DOI: 10.1094/PD-76-1237.

The pathogenicity of Phoma medicaginis var. medicaginis to crowns of alfalfa was determined by inoculating clonal and seedling plants of the cultivar Iroquois with two isolates of the pathogen. Crowns and stubble were inoculated. Crown inoculations with either isolate of P. m. medicaginis caused a black necrosis that extended up into stem bases through one internode and down into upper taproots and occasionally into lateral roots. Wounding of crowns was necessary for infection. Stem stubble provided a suitable infection court for the pathogen to invade crown tissues. Crown and stubble inoculations caused fewer axillary buds, smaller crown diameters and foliar dry weights, and an increased number of dead stems. Pycnidia formed abundantly on dead and dying stubble. P. m. medicaginis was demonstrated to be pathogenic to alfalfa crowns.

Keyword(s): lucerne, Medicago sativa, spring blackstem.