Close examination of infected root ultrastructure showed that the colonization was accompanied by a marked wall modification including primary wall alteration and middle lamella dissolution (Fig. 2B) as compared to the preserved cell-wall architecture in untreated roots (Fig. 2A). The host cells underwent a complete degradation; but the pathogen hyphae did not exhibitany apparent disorganization (Fig. 2C). The observed disruption of root cells coincided with the occurrence of macroscopically visible symptoms (e.g. an important softening of root tissues) leading to plant death.