Influence of mechanical cutting and pathogen application on the performance and nutrient storage of Cirsium arvense

1. In order to develop an effective biological control technique, the influence of mechanical and pathogen treatments on performance and nutrient storage of the weed creeping thistle Cirsium arvense was studied over a 2-year period. To our knowledge, this is the first study to evaluate the effect of a combination of cutting and application of the rust Puccinia punctiformis as a possible biocontrol organism on thistles. 2. We conducted a complete two-factorial design using potted plants, with the application of the rust and cutting of thistles as the treatment factors. Treatments were applied once every year in June. 3. Inoculation reduced above-ground biomass in the first year. First year cutting reduced the number of thistle shoots, but stimulated compensatory growth with an increased growth rate and biomass. Only a second year cutting treatment significantly reduced above- and below-ground biomass, despite an increased growth rate. 4. The number of flower heads was strongly reduced by both cutting and combined treatments. The proportion of fertile flower heads was increased by cutting, whereas the combination treatment strongly reduced it. 5. Analysing storage nutrients, starch concentrations did not differ among treatments. Cutting reduced sugar concentrations, but pathogen infection tended to increase concentrations, resulting in significant interaction effects. Cutting also reduced below-ground carbon concentrations, but the carbon : nitrogen ratio was not influenced. 6. Synthesis and applications. The traditional mechanical technique of cutting was shown to be effective in reducing plant reproductive success and concentration of storage nutrients. Additional infection with a plant pathogen further restricted sexual reproduction, indicating that combined treatments may be more effective for biological control of perennial weed species. Whilst further studies are needed to establish P. punctiformis as a mycoherbicide, some synergism between repeated cutting and rust inoculation indicates that there is potential for a biological approach to the control of creeping thistle.