Pleiochaeta root rot caused by Pleiochaeta setosa is a major threat to the Australian lupin industry. Although Pleiochaeta root rot-resistant Lupinus albus varieties have been bred, there is no information on the likely mechanism of this resistance. Susceptible (Kiev mutant) and resistant (P25758) albus lupin cultivars were inoculated with spores of P. setosa strain PS6-1 and the infection process studied microscopically. No specialised penetration structures were observed on the roots, and infecting hyphae entered roots of both cultivars by growing directly between root surface cells. Lengths of conidial germ tubes on resistant hosts was significantly longer than on susceptible hosts, suggesting that a component of the resistance is via reduced host recognition by the pathogen.