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First Report of Musk Thistle Rust (Puccinia carduorum) in Oklahoma

Musk thistle, Carduus thoermeri (Carduus nutans subsp. leiophyllus), is an important, introduced pasture weed in central and northeastern Oklahoma. Puccinia carduorum was introduced into the United States from Turkey as a potential biological control for musk thistle. P. carduorum has not been reported previously in Oklahoma, thus precluding its field release for biological control research without APHIS approval. There is evidence the organism has moved westward since the initial field studies that began in 1987 in Virginia. In 1994 it was found in Missouri (1). In early November 1997, in Rogers County, Oklahoma, scattered populations of C. thoermeri were found that had moderate to heavy levels of infection with a rust fungus. The pustules contained mostly teliospores; based on teliospore and urediniospore morphology, the fungus was identified as P. carduorum. The morphology and dimensions of urediniospores (21 × 21 μm, avg.) and teliospores (35 × 21 μm, avg.), and the restriction of echinulations to the upper two-thirds to three-fourths of urediniospores, were consistent with P. carduorum. Infection studies with field inoculum were conducted at both Oklahoma State University and USDA-FDWSRU. Rust-infected leaves collected in Oklahoma were air dried and maintained at room temperature for 2 months prior to use as inoculum. Small, symptomless, first-year rosettes of musk thistle were transplanted from the field into a mixture of soil, sand, and peat moss in pots and placed into a growth chamber maintained at 20°C. Seeds of C. thoermeri planted into pots containing the same mixture were maintained in the same chambers. After approximately 6 to 8 weeks, when seedlings and transplants were growing vigorously, both groups of plants were dusted with urediniospores and teliospores from the dried leaves. Inoculated plants were placed either into a 20°C dew chamber for 24 h or were atomized with distilled water, placed into sealed, transparent, polyethylene bags and returned to the 20°C growth chamber for 24 h, after which time the bags were removed. Both sets of plants were then maintained at 20 to 25°C. Chlorotic flecks developed on inoculated leaves after 7 to 8 days; uredinia and urediniospores were present within 10 days after inoculation. Urediniospores from those leaves had the same dimensions and ornamentation pattern as those originally obtained from field collections. A DNA sequence analysis was conducted on the rRNA ITS2 region, which was polymerase chain reaction-amplified from genomic DNA (2) extracted from urediniospores of the Oklahoma isolate grown at FDWSRU. The sequence of the ITS2 region from those urediniospores was identical to the sequence (GenBank accession no. U57351) obtained from the isolate 7803 of P. carduorum from Turkey, used in the Virginia field studies. The confirmed presence of P. carduorum in Oklahoma will enable field research with this rust for management of musk thistle in the state.

References: (1) A. B. A. M. Baudoin and W. L. Bruckart. Plant Dis. 80:1193, 1996. (2) Y. T. Berthier et al. Appl. Environ. Microbiol. 62:3037, 1996.