Interactions of the nonhost French bean plant (Phaseolus vulgaris) with parasitic and saprophytic fungi. I. Fungal development on and in killed, untreated, heat-treated, or blasticidin S treated leaves

Abstract

Germination, penetration, and subsequent growth of four saprophytic and five parasitic fungi nonpathogenic on French bean (Phaseolus vulgaris L. cv. Pinto) were examined on, or in, killed, untreated, and heat-treated or blasticidin S treated leaves in the presence (saprophytes only) or absence of additional nutrients. The saprophytes required either an external supply of nutrients or a diffusate from autoclaved leaves to germinate, and they subsequently did not attempt to penetrate untreated or heat-treated living leaves. In contrast, the parasites germinated well, and penetrated untreated tissue almost exclusively through stomata. For some parasites, preinoculation heat treatment increased the incidence of direct penetration and the degree of fungal growth in the tissue. The saprophytes and nonbiotrophic parasites all penetrated and grew well in leaves that had been killed by autoclaving or freezing. The data suggest that living, intact bean leaves are resistant to the fungal saprophytes tested because these fungi lack pathogenicity factors necessary for germination on, and penetration of, the leaf. Resistance to the parasites, in contrast, appears to reside in heat-sensitive and heat- or blasticidin S insensitive defenses in the leaf that may differ with the fungus.