Field Evaluation of Puccinia carduorum for Biological Control of Musk Thistle

A strain of Puccinia carduorum Jacky, an autoecious rust fungus from Turkey, was evaluated for biological control of musk thistle (Carduus thoermeri Weinmann) in a 3-year field trial. Musk thistle plots were inoculated successfully in the fall and spring of each year. To assess the pathogen′s effect on nontarget species that were slightly susceptible in greenhouse trials, artichoke and selected Cirsium spp. were planted between plots of inoculated musk thistle in two of the years. Only one rust pustule was found on an artichoke plant in 1989; all other plants remained rust-free despite severe disease on surrounding musk thistles, indicating that P. carduorum poses no hazard to these nontarget species. Rust overwintered successfully on musk thistle. Its spread was limited during the rosette stage, and disease became severe only during bolting (stem elongation). There was little difference in plant size between inoculated and noninoculated, fungicide-protected plots, but plant senescence was accelerated and seed production was reduced by 20-57% in inoculated plots. The reduction in seed production in rust-inoculated plots was less than that due to attack by three insects. We cannot rule out the possibility that part of the effects on senescence and seed production may have been due to the fungicides used to protect the noninoculated plots from the rust. However, the reduction in seed production, particularly when in combination with insects, suggests that P. carduorum can contribute to the control of musk thistle.