Expression of resistance to Mycosphaerella pinodes in Pisum sativum

The infection of three resistant and two susceptible inbred lines of Pisum sativum by Mycosphaerella pinodes is described for the first time. Two types of resistance, one expressed in epicotyls and one in leaves, were found in all three resistant lines. On epicotyls of susceptible lines, abundant appressoria and penetrations occurred after a short period of hyphal growth. On epicotyls of resistant lines, hyphae grew extensively but rarely formed appressoria, and these failed to penetrate the cuticle. Attempted penetration was associated with the rapid death of 2–6 epicotyl cells, resembling a hypersensitive reaction. In contrast, resistance of leaves, which was only expressed after penetration, involved localization of the fungus by a mechanism involving delayed leaf cell death. It is suggested that a combination of these two types of resistance might provide effective protection against M. pinodes.