Binary pathways for analysis of primary infection and host response in populations of powdery mildew fungi

Abstract

Events in development of powdery mildew fungi from spore germination to formation of the first haustorium and hypha, including host cell responses, were cataloged using a binary pathway system. This system branches at "event-points" which represent sites of "on–off" alternatives for the occurrence of an event or sites where one of two alternative events occur. The system permits statistical comparisons of probabilities for different events within a given host–parasite combination and for specific events in different host–parasite combinations. Data from the binary pathway system can be converted to percentage of total spores to show the size of the fungal population which passes each event-point, or the data can be resorted to determine how many germlings are associated with a given combination of events.The binary system was used with data for powdery mildew of barley obtained from whole leaves harvested 1 and 2 days after inoculation, which were fixed and then stained with acid fuchsin and aniline blue. Approximately 5000 germinated spores were observed for each host–parasite combination. In compatible combinations, 6–7% of spores failed to produce a normal germ tube and (or) appressorium after germination, and 9–11% of spores failed to produce a penetration peg at the first appressorial lobe. In epidermal cells distant from stomates, only 2–3% of the fungal population produced haustoria and hyphae. The fungus had a high probability of inducing papillae in such cells and a low probability of passing through papillae. In cells near stomates, papillae were less frequent and 46–52% of the population produced haustoria and hyphae.In an incompatible host–parasite combination (with the M1a gene), most penetration pegs terminated in papillae regardless of host cell location. In cells distant from stomates, virtually no germlings grew beyond papillae. In cells near stomates, 11% of the fungal population either penetrated a papilla or entered a host cell without inducing papillae. Of these, about half induced host cell collapse either before or after haustorium formation and half stopped growing after haustorium formation without inducing cell collapse. Thus, incompatability was expressed at several stages in fungal development.