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Phenotypic Comparison of Puccinia carduorum from Carduus thoermeri, C. tenuiflorus, and C. pycnocephalus. W. L. Bruckart, Research plant pathologist, USDA-ARS, Foreign Disease-Weed Science Research Unit, Ft. Detrick, Bldg. 1301, Frederick, MD 21702; G. L. Peterson, biologist, USDA-ARS, Foreign Disease-Weed Science Research Unit, Ft. Detrick, Bldg. 1301, Frederick, MD 21702. Phytopathology 81:192-197. Accepted for publication 6 September 1990. 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, 1991. DOI: 10.1094/Phyto-81-192.

Host range, urediniospore morphology, and isozyme banding patterns of field isolates of rust fungi from Carduus pycnocephalus, C. tenuiflorus, and C. thoermeri, collected in different geographic locations, were compared. Isolates from the three Carduus species are Puccinia carduorum, according to the isozyme data, supporting the view that P. carduorum is preferred to P. cardui-pycnocephali for isolates from C. pycnocephalus. Even though Rogers' Coefficient of Similarity supports the fact that the isolates are the same species, the isolates could be distinguished by isozyme banding patterns for aspartate aminotransferase and glucokinase. Banding patterns for glucose phosphate isomerase, malate dehydrogenase, and mannose phosphate isomerase also enabled distinction between the Carduus rusts and Puccinia carthami from Carthamus tinctorius (safflower). Differences in host preference also were noted for isolates from the three Carduus species. Isolates from C. tenuiflorus and C. thoermeri caused infection most on the species from which they originated, but isolates from C. pycnocephalus were more virulent on C. tenuiflorus. Uredinio-spores from C. pycnocephalus and C. tenuiflorus were significantly (P = 0.05) larger in volume (16 and 12%, respectively) than those from C. thoermeri. Also, the first two species had at least 10% more of the urediniospore surface covered with echinulations than did C. thoermeri.

Additional keywords: biological weed control.