Histopathology of compatibility and incompatibility between oilseed rape and Albugo Candida

Cotyledons of one resistant and three susceptible rape lines/cultivars were inoculated with zoospores of Albugo Candida race 7. Samples of whole cotyledons were examined by differential interference contrast microscopy. The time course of the infection process was followed histologically. Germination of zoospore cysts occurred 2‐3 h after inoculation. Infection was initiated with germ‐tubes penetrating through stomata. Haustorium formation was first observed in the palisade mesophyll cells adjacent to the substomatal chambers 8 h after inoculation. Only after the establishment of the first haustorium did compatible and incompatible interactions begin to differentiate. In the resistant cultivar, most primary hyphae produced single haustoria. Necrosis of the invaded host cell was first observed 12 h after inoculation followed by cessation of fungal growth. The death of host cells was largely restricted to the penetration site; the adjacent non‐penetrated cells remained apparently unaffected. In the susceptible hosts, necrosis of infected cells occurred only infrequently, and hyphal growth continued unabated, resulting in mycelial ramification into the mesophyll. Numerous haustoria were produced. Histological studies showed that the earliest event distinguishing a compatible from an incompatible interaction occurred after formation of the first haustorium and that resistance was not manifested until the host mesophyll cell had come into contact with the first haustorium. The distinction between compatibility and incompatibility was substantiated by quantitative analysis of white rust development on both resistant and susceptible lines/cultivars.