Does disease severity impact on plant foliar chemical and physical responses to two Corymbia citriodora subsp. variegata pathogens?

Eucalypt forests and industrial plantations are threatened by fungal pathogen outbreaks that compromise timber and non-timber products and tree survival. Understanding host-pathogen interactions may contribute to the development of disease management strategies and the selection of resistant genotypes. Resistant and susceptible plants respond differently to disease infection, presenting variations of phenotypic traits, such as leaf physical and chemical parameters that may influence host-pathogen interactions. This study evaluated physical and chemical responses of Corymbia citriodora subsp. variegata to two fungal pathogens, Quambalaria pitereka (a co-evolved pathogen) and Austropuccinia psidii (an established exotic pathogen), using severity of infection as an indicator of plant resistance/susceptibility. Our primary aim was to quantify differential plant responses between uninoculated (controls), susceptible (severely infected) and resistant plants for each pathogen. Plant growth rate, leaf toughness and foliar secondary metabolite profiles of control, resistant and susceptible plants were compared 14 days after pathogen inoculations. Leaf secondary metabolite profiles were analysed from uninfected regrowth of the same plants 90 days after inoculation. The results indicated that only susceptible plants elicited responses to pathogen damage. Susceptible plants infected by Q. pitereka had greater leaf toughness, and susceptible plants infected by A. psidii had reduced growth rates and altered expression of secondary metabolites in comparison to control and resistant plants. Austropuccinia psidii infection led to a reduction in the proportion of monoterpenes and monounsaturated hydrocarbons and an increase in long chain hydrocarbons. No differences in secondary metabolite profiles were found between the treatments 90 days after inoculation, suggesting that differences observed were in response to severe infection and that leaf chemistry is not a good predictive biomarker of susceptibility in C. citriodora subsp. variegata