Role of periderm in resistance of Eucalyptus marginata roots against Phytophthora cinnamomi

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Abstract
Phytophthora cinnamomi Rands invaded phloem tissue preferentially in Eucalyptus marginata Sm. (Jarrah) roots of both primary and secondary structure. Rate of fungal growth in the “primary” roots was less than in roots with secondary thickening. In the main field study jarrah roots between 1.5–2.5 mm in diameter, were inoculated with P. cinnamomi at regular intervals, over 12 months and in most cases invasion of the phloem was halted within the first 8 weeks of infection. In the majority of roots which exhibited secondary structure, lesion shape reflected the pattern of periderm development. Some lesions were peripheral in the outer phloem as cylindrical periderms had formed close to the vascular cambium. Other lesions were limited to a sector of the roots; periderm had formed radially through the secondary phloem and a zone of suberization was often continuous through the cambial zone and xylem. Formation of periderm at the edge of lesions was a multi‐step process with various changes occurring, including lignification and suberization of walls and accumulation of autofluorescent compounds in cell walls and vacuoles. The layers of suberized and lignified cells formed near lesion fronts, were necrophylactic periderms. Necrophylactic periderm formation generally preceded the establishment of periderm of the exophylactic type.