Characterization of the chestnut blight canker and the localization and isolation of the pathogen Cryphonectria parasitica

Abstract

Naturally occurring chestnut blight cankers on Castanea dentata were categorized as virulent (sunken bark with abundant stromata) or hypovirulent (swollen bark lacking stromata). In transverse section, xylem tissue of virulent cankers was discolored and largely nonconductive. Hypovirulent cankers had anomalous secondary xylem tissue that was light in color and conducted safranin dye, although xylem of the main stem axes of hypovirulent cankers was discolored. In naturally occurring cankers, no correlation was found between canker morphology and the presence or absence of double-stranded RNA in the inciting fungus (Cryphonectria parasitica). However, a virulent strain lacking double-stranded RNA induced cankers that resembled natural virulent cankers, and inoculation with a known hypovirulent strain containing double-stranded RNA resulted in swollen cankers that resembled natural hypovirulent cankers. Cryphonectria parasitica was isolated from all growth rings of virulent cankers and from the discolored xylem tissue of hypovirulent cankers but not from the conductive anomalous xylem tissue of hypovirulent cankers. Hyphae were observed via fluorescence microscopy in the xylem of natural and induced virulent cankers but not in hypovirulent cankers, whether natural or induced by artificial means.