Effect of Spore Concentration on the Establishment of Cytoplasmic Hypovirulent (HV), Transgenic HV, and Virulent Isolates of Cryphonectria parasitica, the Chestnut Blight Fungus

Transgenic hypovirulent (HV) strains of Cryphonectria parasitica engineered to contain a complimentary DNA (cDNA) copy of Cryphonectria hypovirus 1 (CHV1) may provide an efficient mechanism to introduce hypoviruses into genetically diverse populations of the chestnut blight fungus. However, elucidating the factors that influence infection of American chestnut (Castanea dentata) and the initiation of HV cankers is necessary to exploit their biological control potential. In this study, inoculation tests were performed to determine whether spore concentration (300 vs. 3,000 spores/inoculation) influenced the incidence of infection after seven months for transgenic HV, cytoplasmic HV and virulent conidia. Inoculations also were conducted using ascospores and mycelium of the virulent and HV strains. Growth and asexual sporulation measurements of the resulting cankers showed that spore concentrations delivered at 300 spores/site had no effect on the number of infections or on canker growth. In contrast, inoculations using 3,000 virulent conidia significantly increased the frequency of infection compared to those made with 300 virulent conidia. Asexual sporulation, however, was not observed on the surface of cankers initiated by virulent, cytoplasmic HV or transgenic HV inocula. Both the cytoplasmic and transgenic HV isolates were less effective at infecting and growing in chestnut bark when compared to the virulent isolate. Deployment of transgenic HV isolates for the biological control of C. parasitica likely provides no significant advantage over control strategies that utilize cytoplasmic HV isolates when canker initiation to establish HV cankers is the goal.