Vol. 54—2000 - NorthEastern Weed Science Society

SPREAD OF TIlE MUSK nnSTLE BIOLOGICAL CONTROL PATIIOGEN Puccinia
carduorum FROM VIRGINIA TO CALIFORNIA.
D. G. Luster, Y. T. Berthier, W. L. Brockart, and M. A. Hack'
ABSTRACT
The rust pathogen Puccinia carduorum, originally collected in Turkey in 1978, was studied in
Virginia in a series of field experiments from 1987-90 for the control of musk thistle (Carduus
thoermerh. Since these studies, rust samples have been collected and examined from musk thistle
populations in several states across the U.S. Urediniospore ornamentation and analysis of
ribosomal Internal Transcribed Spacer 2 (ITS2) DNA sequences wereused to confirm the identity
of the rust samples as P. carduorum. Unique DNA sequences in the ITS2 region of P. carduorum
permit us to discriminate the musk thistle pathogen from a morphologically indistinguishable strain
of P. carduorum that is indigenous to California, and only pathogenic on the closely related
slenderflower thistle (c. tenuiflorus). Teliospores of P. carduorum collected from musk thistle
from California since 1998 were found to contain the same ITS2 sequence as that from the foreign
isolate originally studied in Virginia. These datastrongly suggest that the pathogen has dispersed
across the entire U.S. since the Virginia field studies.
'USDA, ARS, Foreign Disease- Weed Science Research Unit, 1301 Ditto Ave, Ft. Detrick, MD
21702-5023
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