Disease Note

Leaf Spot of Canada Thistle Caused by Alternaria chrysanthemi. D. J. Robeson and G. A. Strobel, Department of Plant Pathology, Montana State University, Bozeman 59717. Plant Dis. 68:251, 1984. Accepted for publication 12 December 1983. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/PD-68-251c.

The susceptibility of Canada thistle (Cirsium arvense (L.) Scop.), an agriculturally important weed, to infection by Alternaria chrysanthemi Simmons and Crosier, the etiologic agent of leaf spot of shasta daisy (Chrysanthemum maximum Ramond), was demonstrated by inoculation experiments and subsequent reisolation of the pathogen from lesions.  Two single-conidium isolates of A. chrysanthemi, ATCC 16028 and 16029, supplied by E. G. Simmons, were used.  Inoculum in the form of mycelial fragments (10⁵/ml) was sprayed on the leaves to the point of incipient runoff.  Florist's chrysanthemum (C. morifolium Ramat. 'Dixie') was also infected by artificial inoculation with A. chrysanthemi.  Discrete, brown, necrotic leaf spots a few millimeters in diameter appeared on all three host species.  Characteristic conidia of the reisolated pathogen were identical with those of A. chrysanthemi.

References: Schmidt, T. Pflanzenschutz 21:13, 1958.  Simmons, E. G. Mycologia 57:140, 1965.