First line of defence: Eucalyptus leaf waxes influence infection by an aggressive fungal leaf pathogen.

Leaf epicuticular waxes provide important anatomical and chemical defences against fungi that infect leaves. In this study we analysed the leaf wax composition of Eucalyptus grandis × Eucalyptus urophylla hybrids with contrasting susceptibilities to Teratosphaeria leaf blight (TLB) caused by Teratosphaeria destructans, one of the most important foliar diseases of Eucalyptus. The Eucalyptus cuticular wax was extracted from non-inoculated and inoculated genotypes with different levels of susceptibility to TLB and analysed by gas chromatography-mass spectrometry. The results showed that a triterpenoid, cycloartenol (CAS), was abundant in a resistant genotype and that hexanedioic acid content increased in the resistant genotypes in response to T. destructans infection. In contrast, palmitic acid was significantly more abundant in the inoculated highly susceptible genotype. In-vitro and in-planta T. destructans spore germination assays with pure compounds, showed that CAS and hexanedioic acid significantly inhibited spore germination. Application of these two compounds to the leaves of a susceptible host also significantly increased resistance to infection. In contrast, palmitic acid promoted spore germination and, when applied to the leaves of a resistant genotype, increased colonization by the pathogen. This is the first study providing insights into differences in the leaf wax composition of hosts with different levels of susceptibility to T. destructans. It also showed that leaf wax compounds can modulate spore germination and, ultimately, host resistance to infection.