Host responses to natural infection by Cytonaema sp. in the aerial bark of Eucalyptus globulus

The chemical and anatomical host responses to natural fungal infection by Cytonaema sp. in the aerial bark of 3‐year‐old Eucalyptus globulus plantation trees were examined. The lesion margin (LM) of the canker‐infected bark was characterized by the formation of a layer of dark extractives visible to the naked eye. Chemical analysis of the LM by gas chromatography‐mass spectrometry (GC‐MS), high‐performance liquid chromatography (HPLC) with ultraviolet detection and HPLC‐MS using negative ion electrospray ionisation indicated the presence of a range of compounds including hydrolysable tannins, polymeric proanthocyanidins, flavonoid glycosides, formlyated phloroglucinol compounds and volatile terpenes. These compounds were either undetectable in healthy tissue or present at significantly lower concentrations than in the LM. The LM of the canker‐infected bark was morphologically distinct from healthy phloem, its characteristics varying depending on severity of canker infection. In superficial infections in which only the phloem was affected, the following LMs were observed: (i) a continuous wound periderm of multiple layers, or (ii) an incompletely differentiated and discontinuous wound periderm. In cases of severe canker infections in which the vascular cambium had been killed, the new phloem formed subsequently contained traumatic oil glands in addition to the responses observed for superficial canker infections. All LMs were characterized by the formation of new parenchyma cells that stained positive for the presence of polyphenols. The significance of the chemical and structural responses as defence mechanisms against fungi causing stem canker is discussed.