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Infection of Musk Thistle by Puccinia carduorum Influenced by Conditions of Dew and Plant Age. D. J. Politis, Research Associate, Department of Plant Pathology, Pennsylvania State University, University Park 16802. W. L. Bruckart, Research Plant Pathologist, USDA, ARS, Plant Disease Research Laboratory, Frederick, MD 21701. Plant Dis. 70:288-290. Accepted for publication 23 September 1985. 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, 1986. DOI: 10.1094/PD-70-288.

Urediniospores of Puccinia carduorum infected musk thistle (Carduus nutans) over a wide range of dew periods and temperatures and caused significant stress to rosettes. Infection resulted after 4 hr of dew at 14 C and after 8 hr of dew at 8 C, but optimal conditions of dew period and temperature were 12 hr between 17 and 24 C. Rosettes up to 10 wk old when first inoculated were very susceptible to infection; rosettes 2 wk old were most susceptible, and susceptibility declined with plant age according to the quadratic estimation Y = 7.95 1.27 X + 0.06 X2 (adj. r2 = 0.60). Inoculation of individual 4-wk-old plants once per week for 4 wk resulted in a reduction of rosette and root dry weights of 64 and 74%, respectively, compared with the controls. The conditions of dew and temperature needed for infection of musk thistle by P. carduorum are prevalent in much of North America.

Keyword(s): biological weed control, rust disease, rust fungi, Uredinales.