Effects of Hypovirulence inCryphonectria parasiticaand of Secondary Blight Infections on Dieback of American Chestnut Trees

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Abstract
Compatible hypovirulent (H) isolates of Cryphonectria parasitica applied to either wounded or nonwounded chestnut blight cankers reduced dieback of American chestnut trees 27-51 mo. after inoculation in comparison to dieback of wounded or nonwounded controls. Most control trees were killed back to the point of inoculation with virulent (V) isolates within 15-19 mo. after inoculation. Initially, H treatments of wounded cankers promoted healing at the inoculation point, but subsequently the treatments sometimes failed to stop girdling by naturally developing basal cankers or secondary infections by C. parasitica at the inoculation point. Thirty-seven months after initial treatment, H isolates were recovered from a higher percentage of live treated trees than dead treated trees and were not recovered from dead control trees. In a 2nd experiment, H isolates were recovered more frequently from nonwounded treated cankers than from wounded treated cankers. Secondary infection of cankers was demonstrated by recovery of V isolates of C. parasitica that differed from V isolates used for inoculum in vegetative compatibility (v-c) and/or in conversion susceptibility.