Two strains of the fungusMagnaporthe griseacaused responses in goosegrass, weeping lovegrass, and rice that varied from no visible symptoms, through isolated discrete lesions with or without brown margins, to coalescing lesions that completely killed the inoculated areas of the leaf. The fungus sporulated under conducive conditions in all of the five plantndash;strain combinations that produced visible symptoms, but the degree of sporulation varied considerably. Microscopical examination revealed a previously unreported dimorphism in the infection hypha in all three plant species. In all combinations, some appressoria failed to form infection hyphae. In all but one combination, some infection hyphae failed to differentiate and (or) became restricted to a single epidermal cell with autofluorescent walls. Quantitative data showed that the presence and density of lesions reflected the frequency with which appressoria formed growing fungal colonies; lesion appearance depended on the interaction between such colonies and the leaf tissue. The results suggest that the different plantndash;strain combinations represent a continuum of plantndash;fungus interactions, and that small differences in the frequency of different plantndash;fungus interactions at the cellular level may have significant effects on macroscopic symptom development.
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