Cirsium arvense, Canada thistle, is one of the most detrimental and invasive weeds for agricultural production and rangeland health. It is the most commonly listed noxious weed in the US with a clonal, resilient root system that makes it difficult to eradicate. Insect biocontrol agents used in lieu of potent herbicides have proven ineffective. A fungal pathogen, Puccinia punctiformis or CT-rust, systemically infects and kills only Canada thistle. However, CT-rust rarely reaches epidemic proportions in natural populations. Insect herbivory could augment rust infection by altering plant defense hormones. Jasmonic acid (JA) is up regulated following herbivory and acts in opposition to salicylic acid (SA) which induces defense responses against pathogens. The objective of this study was to determine if manipulating plant defense hormones affects host susceptibility to rust infection and ultimately make CT-rust a more effective biocontrol agent. A foliar spray of JA, SA, or water was applied to greenhouse grown thistle plants for three weeks following inoculation with CT-rust teliospores. Water foliar sprayed, uninoculated thistles acted as a control. We visually inspected plants for systemic infection and after four months we harvested the plants for above and below-ground biomass and to test roots for latent infection.
The rate of symptomatic infection in the above ground biomass was 0% for the uninoculated control and 58.3% JA, 8.3% SA, and 16.7% water treated plants. Only secondary and tertiary shoots exhibited symptomatic infection. Inoculated thistles also had up to a 22% reduction in root biomass. The increased rate of symptomatic infection in plants inoculated with a JA treatment indicates that a boost of JA at the time of inoculation reduces SA-dependent defense responses, and increases plant susceptibility to CT-rust infection. Therefore, it is likely the efficacy of CT-rust would increase when used in conjunction with other insect biological control agents as JA is released in plants in response to insect herbivory. Additionally, this study is the best current evidence that there are latent infections and that these are detrimental to Canada thistle as seen in the reductions in root biomass even without visible symptoms of infection.