Host-parasite relations betweenAllomyces andRozella

Summary Zoospores of the obligately parasitic chytridRozella allomycis encyst all over the hyphae of a susceptible host, the water-moldAllomyces arbuscula, but many of them fail to penetrate. At the sites of successful penetration lomasomes occur, and the inner layer of the host cell-wall grows and invaginates around a papilla, through the center of which the parasite enters. This host response resembles instances of normal, localized inward growth of fungal cell-walls. The response may also be related to a defense reaction of walled cells which form callosities to block the invasion of fungi.Rozella appears to utilize this response and to depend upon it—probably in order to enter the host cell with minimal disruption. A similar relationship may control the penetration of various obligately parasitic plant-pathogens into their hosts.