Histological Changes in the Roots of an Avocado Cultivar, Duke 7, Infected withPhytophthora cinnamomi

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Abstract
Roots of the moderately resistant avocado [Persea americana] cultivar, Duke 7, formed limited lesions when inoculated with the fungal pathogen, Phytophthora cinnamomi. Histological examination of tissue changes revealed two distinct anatomical responses at the lesion boundary; the formation of necrophylactic periderm in the cortex and whorls of cells walling off infected phloem bundles in the stele. Necrophylactic periderm divided necrotic, infected tissue from nonnecrotic, uninfected cortical tissue from the epidermis through to the endodermis and always coincided with the very rapid disappearance of infected cortical tissue. Phloem bundles, the site of intensive hyphal colonization in lesioned tissue, were devoid of hyphae after the formation of cell whorling and cell wall thickening at the lesion boundary. These anatomical responses appear to be an important component in determining the resistance response of this rootstock.