Susceptibility of Carduus, Cirsium, and Cynara species artificially inoculated with Puccinia carduorum from musk thistle

control of Carduus thoermeri under controlled greenhouse and field conditions. We measured the susceptibility of 16 accessions of C thoermeri (the target weed), 10 accessions of related weedy Carduus spp,, 22 native and 2 weedy Cirsium species, and Cynara scolymus (artichoke). All collections (including 99.2% of the individuals) of C. thoermeri were very susceptible, as were collections of putative hybrids between C. thoermeri and other large-flowered Carduus spp. Disease incidence on Carduus nutans or Carduus macrocephalus was between 0 to 5% of plants inoculated. Of the 454 individual Cirsium thistles inoculated, 83% did not develop symptoms (= immune) at any stage of growth, including all individuals in 11 species, For the Cirsium species in which symptoms were observed, 75% were immune when inoculated 4-5 weeks after planting, and no susceptible reactions developed after reinoculation of these individuals 2 weeks later. A similar effect of plant age and susceptibility was noted for Cy. scolymus. Results similar to those in the greenhouse occurred also from field inoculations of eight selected Cirsium spp., two Cynara spp., and three Carduus spp. in Switzerland. All individuals of C. thoermeri were susceptible; ratings for disease severity were between ''2'' and ''4'' (the maximum rating), For the remainder of the species, the highest rating was ''1'' for Cirsium and Cynara spp. and ''2'' for the other Carduus spp, Despite the symptoms on species other than C thoermeri, P. carduorum was considered low risk because infections on these species were weak and the pathogen could not be maintained on any nontarget species under optimal greenhouse conditions. Data from these experiments were part of a proposal to the USDA, Animal and Plant Health Inspection Service (APHIS), for use of P. carduorum in the United States for biological control. Permission was granted by APHIS and the Virginia Department of Agriculture in 1987 to conduct a field study of P. carduorum near Blacksburg, Virginia.