Susceptibility ofCarduus, Cirsium,andCynaraSpecies Artificially Inoculated withPuccinia carduorumfrom Musk Thistle

Puccinia carduorumfrom Turkey was evaluated for biological control ofCarduus thoermeriunder controlled greenhouse and field conditions. We measured the susceptibility of 16 accessions ofC. thoermeri(the target weed), 10 accessions of related weedyCarduusspp., 22 native and 2 weedyCirsiumspecies, andCynara scolymus(artichoke). All collections (including 99.2% of the individuals) ofC. thoermeriwere very susceptible, as were collections of putative hybrids betweenC. thoermeriand other large-floweredCarduusspp. Disease incidence onCarduus nutansorCarduus macrocephaluswas between 0 to 5% of plants inoculated. Of the 454 individualCirsiumthistles inoculated, 83% did not develop symptoms (= immune) at any stage of growth, including all individuals in 11 species. For theCirsiumspecies 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 forCy. scolymus.Results similar to those in the greenhouse occurred also from field inoculations of eight selectedCirsiumspp., twoCynaraspp., and threeCarduusspp. in Switzerland. All individuals ofC. thoermeriwere susceptible; ratings for disease severity were between “2” and “4” (the maximum rating). For the remainder of the species, the highest rating was “1” forCirsiumandCynaraspp. and “2” for the otherCarduusspp. Despite the symptoms on species other thanC. thoermeri, P. carduorumwas 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 ofP. carduorumin the United States for biological control. Permission was granted by APHIS and the Virginia Department of Agriculture in 1987 to conduct a field study ofP. carduorumnear Blacksburg, Virginia.