SUSCEPTIBILITY OF CARDUUS, CIRSIUM, AND CYNARA SPECIES ARTIFICIALLY INOCULATED WITH PUCCINIA-CARDUORUM FROM MUSK THISTLE

control of Carduus thoermeri under controlled greenhouse and field con ditions. We measured the susceptibility of 16 accessions of C thoermer i (the target weed), 10 accessions of related weedy Carduus spp,, 22 n ative and 2 weedy Cirsium species, and Cynara scolymus (artichoke). Al l collections (including 99.2% of the individuals) of C. thoermeri wer e 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 plantin g, and no susceptible reactions developed after reinoculation of these individuals 2 weeks later. A similar effect of plant age and suscepti bility was noted for Cy. scolymus. Results similar to those in the gre enhouse occurred also from field inoculations of eight selected Cirsiu m spp., two Cynara spp., and three Carduus spp. in Switzerland. All in dividuals of C. thoermeri were susceptible; ratings for disease severi ty were between ''2'' and ''4'' (the maximum rating), For the remainde r of the species, the highest rating was ''1'' for Cirsium and Cynara spp. and ''2'' for the other Carduus spp, Despite the symptoms on spec ies other than C thoermeri, P. carduorum was considered low risk becau se infections on these species were weak and the pathogen could not be maintained on any nontarget species under optimal greenhouse conditio ns. Data from these experiments were part of a proposal to the USDA, A nimal and Plant Health Inspection Service (APHIS), for use of P. cardu orum in the United States for biological control. Permission was grant ed by APHIS and the Virginia Department of Agriculture in 1987 to cond uct a field study of P. carduorum near Blacksburg, Virginia.