Light microscope observations of susceptible, host resistant, and nonhost resistant interactions of alfalfa withPhytophthora megasperma

摘要

Susceptibility and host resistance in alfalfa (Medicago sativaL.) toPhytophthora megaspermaDrechs. f. sp.medicaginis(Pmm), an alfalfa pathogen, and nonhost resistance toP.megaspermaf. sp.glycinea(Pmg), a pathogen of soybean (Glycine max(L.) Merr.) but not alfalfa, were compared by light microscopy. Zoospores of Pmm and Pmg were attracted to and became encysted upon roots of Pmm-resistant and Pmm-susceptible alfalfa seedlings within 60ensp;min after inoculation. Cysts germinated and grew toward the roots, and penetration was usually intercellular. The extent of colonization by Pmm and Pmg was similar in all cases at 2ensp;h after inoculation, but at 12 and 24ensp;h colonization was much more extensive in the susceptible interaction than in the host resistant or nonhost resistant interactions. In both the host resistant and nonhost resistant interactions, root cells in contact with hyphae were plasmolyzed and (or) necrotic. Although numerous similarities exist between host resistance to Pmm and nonhost resistance to Pmg in alfalfa seedlings, nonhost resistance is apparently triggered more rapidly and (or) is more effective than host resistance in this system.