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Pathogenicity and Canker Control by Mixtures of Hypovirulent Strains of Endothia parasitica in American Chestnut. Richard A. Jaynes, Geneticist, Connecticut Agricultural Experiment Station, New Haven 06504; John E. Elliston, assistant Plant Pathologist, Connecticut Agricultural Experiment Station, New Haven 06504. Phytopathology 70:453-456. Accepted for publication 2 November 1979. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-453.

Pathogenicity of hypovirulent strains of Endothia parasitica on American chestnut stems decreased dramatically when two or more were mixed. Probably this effect is due to the exchange of genetic hypovirulence factors between strains possessing different factors; ie, superinfection, and fungal cell death that results from the interaction of strains that are vegetatively incompatible. Pathogenicities of normal and hypovirulent strains of Endothia parasitica were highly variable and strain dependent. Single hypovirulent strains often have been ineffective in controlling cankers, presumably because vegetative incompatibility hinders transfer of factors that confer hypovirulence. Mixtures of different hypovirulent mycelia effectively overcame vegetative incompatability and rapidly arrested canker development in both laboratory and field tests. However, mixtures tested to date may be too debilitated to survive and disseminate in the wild.