Initiating a plant herbivory response increases impact of fungal pathogens on a clonal thistle

Abstract

Cirsium arvense, or Canada thistle, is one of the most detrimental weeds for agricultural production and rangeland health. The autecious 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 the addition of JA at the time of inoculation increases the infection rates, both the incidence and severity, and impact of CT-rust which could make it a more effective control agent for Canada thistle. In the first greenhouse experiment, I 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 water. We also found that, while JA does have a slight effect on plant growth, applying herbivory defense hormones does augment CT-rust's action as a biological control.