The role of ascospores and conidia as propagules in the disease cycle of Hypoxylon mammatum

Two lines of evidence, population structure and conidial germination properties, support the hypothesis that ascospores and not conidia disperse the fungus in the disease cycle of hypoxylon canker of aspen. Mycelial interactions, identified by the formation of a conidial interface between genetically different isolates, provided phenotypic markers for determining population structure in Hypoxylon mammatum. Single ascospore isolates from 14 asci from six perithecia found on one canker were tested for interactions. Each ascus contained four different interaction groups composed of identical twin ascospore isolates, indicating the heterothallic production of asci within perithecia. No isolates of the same group were found in interascus pairings. Mycelial isolations from cankers showed that each canker consisted of a single interaction type, suggesting its origin from a single spore. No isolates of the same type were found among cankers on adjacent trees, either in a wild clone or in a garden plantation, indicating that cankers caused by asexual propagation of the fungus were rare. Conidia produced by the mycelial interaction of two different isolates germinated 72-80% as judged by germ tube emergence. Filtering conidial preparations through glass wool removed most of the hyphal fragments. Colony formation by these preparations was only 5% of the estimated number of conidia plated, but two to eight times more than the estimated number of hyphal fragments contaminating the preparations. Germinating conidia produced a few short hyphal branches on the surface of the agar medium and one long aerial hypha with branches at its tip, but no further development occurred. We concluded that conidia do not serve primarily as asexual propagules. We also observed two forms of conidiogenous cells, geniculate, and nodulose. The propensity to produce these differed among ascospore isolates. Conidial sizes varied significantly among isolates, and the conidia produced at the interface of opposed mycelia were usually significantly different from one or both of the parental strains.