Cytological observations of early infection process by Fusarium oxysporum f. sp. radicis-lycopersici in tomato plants

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Abstract
A survey of the initial infection phases of Fusarium oxysporum Schlecht. f. sp. radicis-lycopersici Jarvis and Shoemaker in tomato roots demonstrated that the epidermis was colonized from 12 to 24 h after inoculation. Until 96 h the pathogen was usually limited to the outer cortical area, where fungal cells were found to be either intercellular or intracellular. Host cell wall thickenings and papilla formation were noticeable in the cortical cells but totally absent in the endodermis and the vascular stele. In the cortical area, cytoplasm and walls of affected host cells were mostly disintegrated when the whole root tissues were colonized about 144 h after inoculation. Between 96 and 120 h, hyphae were visible in the endodermis, and 24 h later the vascular stele was colonized. In the latter area, parenchyma cells generally reacted as in the inner cortical area and invasion of vessels proceeded directly through middle lamella and pit membranes. When colonized, vessels contained fibrillogranular material interspersed with bubbles and an osmiophilic coating material. This coating material lined the vessel secondary wall and pit cavities and appeared thicker in the more occluded vessels. The possible implications of these observations on symptoms expression in this and similar plant diseases are discussed.

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