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• Manipulating plant defense hormones increases the impact of fungal pathogens. • Jasmonic acid application increased the impact of CT-Rust on Cirsium arvense. • This effect was seen for belowground but not aboveground biomass. Cirsium arvense, or Canada thistle, is one of the most detrimental weeds for agricultural production. Native to south-eastern Europe, the species is now found in Many parts of Asia, North America and Australasia. In North America and New Zealand the species is considered invasive, displacing native vegetation and reducing the quality of forage in rangelands and pastures. The autoecious fungus, Puccinia punctiformis , or CT-rust, shows potential as a control agent but rarely reaches epidemic proportions in natural populations. Manipulating plant defense hormones could alter host susceptibility and allow CT-rust to have more widespread impact. To determine if applying hormones increases the infection by the fungal pathogen, Canada thistle plants were inoculated and sprayed with jasmonic acid (JA) and salicylic acid (SA). Results show that jasmonic acid application interacted with inoculation, increasing infection rates, both incidence and severity, and impact of CT-rust. We found that JA increased infection rates by nearly 20%. Infection consistently reduced root biomass and this reduction was 45% greater with the addition of JA compared to untreated control. Addition of JA at the time of inoculation could make it a more effective control agent for Canada thistle. [ABSTRACT FROM AUTHOR]