Studies of plant interactions with pests and pathogens in Corymbia species and hybrids

Flavia Sarti Bonora

doi:10.25907/00479

Pests and pathogens are a major concern in eucalypts in native forests and plantations, as they cause loss of biodiversity and productivity and reduce tree survival. Pest and pathogen management requires multiple strategies to minimise impacts and maximise forest health, involving silvicultural practices, use of pesticides, biocontrol and selection of resistant genotypes through breeding programs. Greater knowledge of host interactions with pests and pathogens and consequences for plant performance and physiology may contribute significantly to the development of pest and disease management strategies. This thesis focuses on plant interactions with pests and pathogens and understanding plant physical and chemical parameters in response to damage. The research included controlled and field experiments with Corymbia spp. and hybrids, ecologically and economically important eucalypt taxa. The research investigated: (1) Corymbia citriodora subsp. variegata chemical and physical response to insect damage; (2) Differences in physical and chemical foliar parameters between C. citriodora subsp. variegata plants resistant and susceptible to Quambalaria pitereka and Austropuccinia psidii; (3) Comparison between C. citriodora subsp. variegata response to Q. pitereka and A. psidii infection; (4) The levels of susceptibility of Corymbia species and hybrids to pests and pathogens in the field and the consequences of damage on plant performance; The three controlled experiments were conducted in a shadehouse to examine specific plant responses to pest and pathogen damage, involving analyses of plant growth rate, leaf anatomy and leaf chemistry. In the first experiment, seedlings of C. citriodora subsp. variegata were subjected to Paropsis atomaria larval feeding, mechanical wounding and no damage (control), to examine the effect of leaf tissue removal on the growth rate, leaf toughness, leaf trichome density and leaf secondary metabolites. Plant response was analysed in damaged leaves (local response), undamaged leaves in damaged plants (systemic response), and in new leaves produced post-treatment (delayed response). Plants did not exhibit a local chemical response, nor differences in growth rate or measured leaf physical parameters following damage. There was, however, a systemic chemical response to P. atomaria larval feeding and mechanical wounding, with plants subjected to larval feeding also exhibiting a delayed chemical response. Six compound classes were identified, of these, long-chain hydrocarbons were lower in the treatments relative to undamaged plants, whereas the proportion of monounsaturated hydrocarbons and monoterpenes was higher. When analysed across compound classes, larval mortality was significantly correlated with monounsaturated hydrocarbons and long-chain hydrocarbons. The potential mechanisms underlying the observed responses are discussed. The second shadehouse trial examined plant-pathogen interactions. Corymbia citriodora subsp. variegata seedlings from different provenances were inoculated with either Q. pitereka or A. psidii, and resistance classes for the two pathogens were determined. Differential plant responses between severely infected (susceptible) and resistant plants (low levels of damage) for each pathogen were quantified. Growth rate, leaf toughness and leaf secondary metabolites were compared between uninoculated (control), resistant and susceptible plants for both pathogens. Post-treatment leaf samples were taken when plants were free of disease, to determine if secondary metabolite profiles would assist in susceptibility prediction. Susceptible plants infected by Q. pitereka had greater leaf toughness while those infected by A. psidii had reduced plant growth and changes in the expression of secondary metabolites in comparison to uninoculated controls and resistant plants. The plants severely damaged by A. psidii exhibited a reduction in the proportion of monoterpenes and monounsaturated hydrocarbons and an increased proportion of long chain hydrocarbons. Post-treatment samples did not differ from each other or control plants, suggesting secondary metabolites are not a good predictor of susceptibility to these pathogens in C. citriodora subsp. variegata. The potential mechanisms underlying the observed responses are discussed and contrasted with similar studies in other species. The third shadehouse trial involved seedlings of C. citriodora subsp. variegata from Woondum (26° 25' S, 152° 81' E), assome families from this provenance may display resistance to Q. pitereka and susceptibility to A. psidii. . Quambalaria pitereka innoculated plants, A. psidii inoculated plants and uninoculated control plants were assessed to compare phenotypical responses including growth rate, leaf toughness, leaf thickness, leaf chemistry and leaf histochemistry. Only A. psidii infection resulted in alteration of leaf toughness and leaf chemistry in comparison to Q. pitereka infection and controls. The histochemical analyses suggest that both pathogens alter the distribution of polyphenols and tannins in comparison to control plants. Further discussion underlying the differences between infections by these pathogens are explored. The field experiment studied four pure taxa – C. citriodora subsp. citriodora, C. citriodora subsp. variegata, C. henryi and C. torelliana – and three hybrids – C. torelliana × C. citriodora subsp. citriodora, C. torelliana × C. citriodora subsp. variegata and C. torelliana × C. henryi. The aim of the field trial was to examine tree performance, leaf ontogeny and susceptibility to pests and pathogens over time, through a series of assessments conducted over 24 months. Throughout the experiment, plants were naturally affected by several generalist insect defoliators, and Q. pitereka, an endemic pathogen of Corymbia spp. that causes necrosis, leaf distortion, and affects plant performance and survival. Results indicated significant variations between taxa and across time for all assessed parameters and significant taxon × time interactions. The characteristics of each taxon and the influence of environmental conditions underlying the studied parameters of tree performance and pest and disease susceptibility are discussed. These results reveal the effects of damage on tree performance, the variable susceptibility to pests and pathogens between Corymbia species and hybrids, and suggest mechanisms underlying leaf physical and chemical responses observed in C. citriodora subsp. variegata under pest and pathogen damage. The knowledge generated in this thesis benefits the development of pest and disease management, contributes to strategies to select resistant genotypes and adds a relevant understanding on plant responses and interactions with pests and pathogens in natural and controlled conditions.

Studies of plant interactions with pests and pathogens in Corymbia species and hybrids

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