Bayoud disease of date palm: ultrastructure of root infection through pneumatodes

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Abstract
Young axenically grown plants of date palm, Phoenix dactylifera cv. Deglet-nour, were inoculated with Fusarium oxysporum Schl. f. sp. albedinis (Killian et Maire) Gordon, using conidia applied either on the pneumatode-free main root or on the pneumatodes of rootlets. After incubation for 15 days, infection was successful only in the latter case. The fungus grew inside the aerenchyma and clung tightly to the cell walls, the wartlike outgrowths, and the matrix present in the intercellular spaces. These structures were more or less altered in the presence of the hyphae. The fungus proceeded both intercellularly and intracellularly through the cortex. A dense hyphal sheet was observed facing the suberized endodermis, but the fungus somehow made its way into the stele. Penetrations of thick-walled cells commonly occurred through pits. Root colonization was slower toward the root apices than toward the base. After 15 days, the fungus was present at least 8 cm above the inoculation point at the base of the root.