Strategies for Enhancing Dissemination of Hypovirulence in Endothia Parasitica: State of the Art

ABSTRACT.--The chestnut blight epidemics of eastern United States and Italy were compared. In spite of substantial differences in their ecosystems (host species, forest community, rainfall distribution, major soil types), the epidemics caused by virulent (V) strains of Endothia parasitica proceeded apparently at similar rates, i.e. ca. 18 and 23 miles per year for eastern United States and Italy, respectively. This was facilitated by the abundant production of conidia, which are spread by a variety of agents, and windblown ascospores. Healing cankers attributed to hypovirulent (H) strains of the pathogen were observed in Italian chestnut forests 12 to 15 years after attack by V strains. Within 20 to 30 years, H strains succeeded in checking blight in Italy. The rate of spread of H strains in Italy, therefore, was about the same as that for V strains. In the absence of significant spread of H strains in the eastern United States during the past 80 years, it seems unlikely that such spread will occur naturally in the foreseeable future. It may, however, be possible to establish disseminating H strains by knowledgeable intervention. Perhaps the best hope for enhancing dissemination of H strains lies in an understanding of how hypovirulence spreads where it is spreading. Intensive studies in these areas (i.e. Italy, France, and perhaps most significantly, western Michigan) should seek to determine rates and patterns of spread, rates of increase of H strains, host density, incidence of vectors or other novel relationships affecting dissemination, contribution of asexual and sexual sporulation, and inoculum density of H strains. Slow rates of local spread need not preclude success as H strains can be established artificially in many places throughout the natural range of American chestnut. Chestnut blight continues unabated, with rare possible exceptions,in the eastern United States while the epidemic in southern Europe has sub-sided. This has provided hope that chestnut blight can be controlled eventually in the United States. An understanding of how hypovirulent (H) strains of Endothia parasitica were disseminated in Europe may bear heavily on the outcome of efforts to combat blight in North America. To address this question, it seems appropriate to first compare the epidemics of North America and Europe with particular emphasis on factors which could have influenced the dissemination of virulent (V) and H strains. This information could be instructive as we explore possible strategies to enhance and detect dissemination of H strains within the natural range of American chestnut.

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Author(s): Louis Shain

Publication: American Chestnut Proceedings - 1982