Date of Graduation


Document Type


Degree Type



Davis College of Agriculture, Natural Resources and Design



Committee Chair

William L MacDonald


Variable recovery of hypovirulent (HV) and virulent (V) isolates from repeatedly sampled, identical chestnut blight cankers, caused by Cryphonectria parasitica, prompted this study to monitor more closely the isolate recovery over time to determine hypovirulent or virulent status. To meet the objective, laboratory and field experiments were conducted between 1998 and 2000 to recover isolates from cultures and cankers challenged with HV inoculum. Cultural studies demonstrated (1) hypovirus transfer readily occurred to actively growing colony margins, and (2) that established, virulent mycelium one-to-six weeks-old at time was unable to acquire hypovirus. Results from field studies demonstrated a significant difference in recovery of HV isolates obtained from older portions versus younger portions of cankers. The greatest recovery of HV isolates was from mycelium established just after challenge (44.4%) versus 4.6% of samples HV from the portions of cankers established four months prior to challenge. When bark plugs were sub-divided into different tissues, 20% contained both virulent and hypovirulent mycelium. Based upon this work there appears to be at least one hypothesis to explain variable recovery of HV and V isolates from identical cankers sampled over time: that rather than conversion of existing mycelium to HV status, recovery of HV isolates indicates that the subsequent fungal growth happened to be hypovirulent.