Project Director

Craddock,J. Hill

Department Examiner

Kovach, Margaret; Aborn, David; Lynch, John


Dept. of Biological and Environmental Sciences


University of Tennessee at Chattanooga

Place of Publication

Chattanooga (Tenn.)


The American chestnut tree (Castanea dentata) was reduced from an important canopy hardwood to an understory shrub by the chestnut blight fungus (Cryphonectria parasitica). Hypovirulence, a phenomenon whereby Cryphonectria parasitica is weakened in its ability to incite chestnut blight, is caused by doublestranded RNA (dsRNA) molecules of viral origin in the cytoplasm of the fungus.Hypovirulence serves as a biological control of C. parasitica in Europe and small, isolated areas in North America, but has not been an effective, widespread biological control in North America. It has been hypothesized that the moderately higher levels of host resistance of the European chestnut tree (Castanea sativa) to C. parasitica allow for a stronger expression of hypovirulence as compared with the extremely blight-susceptible C. dentata. The present study attempts to test this hypothesis using a population of half-sibling BC2F2 C. dentata hybrids. This population was screened for blight resistance in 2003 using a virulent strain of C. parasitica (Ep155) and shown to segregate into susceptible, intermediate, and resistant classes. The trees were also inoculated with two hypovirulent strains of C. parasitica at the time of the screening. This population represented a unique opportunity to observe the differential expression of hypovirulence in a population of trees of known blight resistance. Survivorship within each resistance category was recorded and compared. Canker health and abundance of stromata were determined for the Epl55 CHV1- Euro7 (hypovirulent) canker on each tree. Additionally, samples from the Ep155 CHV1-Euro7 canker on the six most resistant trees were collected and assayed for the presence or absence of dsRNA. Survivorship within the resistant trees was much greater than the susceptible trees. There was also a strong tendency for resistant trees to express hypovirulence much more robustly than susceptible trees. Doublestranded RNA was recovered from one experimentally incited canker. In general,these data support the hypothesis that some host resistance is requisite for the expression of hypovirulence.


I would like to thank, first and foremost, Dr. J. Hill Craddock for supervising this project, and for his guidance and friendship. I would also like to thank my committee members and Mark Double at West Virginia University for their time and expertise. Mark Alexander, Stephen Alexander, and Shannon Cagle assisted with data collection and provided friendship and support throughout the project. I give a special thank you to my father, Paul D. Worthen, whose love and encouragement made this project possible.


B. S.; An honors thesis submitted to the faculty of the University of Tennessee at Chattanooga in partial fulfillment of the requirements of the degree of Bachelor of Science.




American chestnut--Disease and pest resistance--Genetic aspects; Selection (Plant breeding); Chestnut blight


Natural Resources and Conservation

Document Type



v, 75 leaves





Call Number

LB2369.5 .W677 2005