Date of Graduation

1998

Document Type

Thesis

Degree Type

MS

College

Davis College of Agriculture, Natural Resources and Design

Committee Chair

William L. MacDonald

Committee Member

Dennis W. Fulbright

Committee Member

Ray R. Hicks

Committee Member

Daniel G. Panaccione

Abstract

Double stranded RNAs (dsRNAs) in the virus family Hypoviridae can be effective agents for the biological control of chestnut blight, however, a vegetative compatibility system in Cryphonectria parasitica is a major barrier to hypovirus transmission in the diverse populations of this fungus in North America. Hypoviruses are carried in the cytoplasm of the fungus and can be transmitted by cytoplasmic exchange during hyphal anastomosis. Anastomosis in C. parasitica is controlled by at least five biallelic vegetative incompatibility genes (vic genes). Laboratory results show that heteroallelism at some vic genes is more likely to inhibit hypovirus transmission than heteroallelism at other vic genes. This field study was undertaken to evaluate the effects of vic 1 and vic 2 on the horizontal transmission of hypoviruses between strains of Cryphonectria parasitica. To accomplish this, virulent cankers were established on chestnut trees in October 1995, using strains of known vegetative compatibility. One year later the virulent cankers were exposed to hypovirulent strains with which they were vegetatively compatible, heteroallelic at vic 1, heteroallelic at vic 2, or heteroallelic at vic 1 and vic 2. Two different hypoviruses were used, and hypovirulent inoculum was delivered via an inoculated bark patch mounted above the infection. A reciprocal pairing, in which the roles of the virulent and the hypovirulent strains were reversed, was established for each pairing to identify cases of epistasis or unidirectional transmission. Hypovirus transmission was evaluated in November 1996, February 1997 and May 1997 by culturing bark plugs from cankers and scoring the resulting colonies by pigmentation, morphology and dsRNA content. Reciprocal transmission of both hypoviruses was observed when strains were homoallelic. When strains were heteroallelic at vic 1 transmission of both hypoviruses was unidirectional, occurring when the canker inciting strain possessed the allele vic 1-1 and the hypovirulent strain possessed the vic 1-2 allele. However, in most cases when transmission occurred, the sampling location from which hypovirulent isolates were recovered within cankers was variable. Neither hypovirus was transmitted when canker inciting and inoculum strains were heteroallelic at vic 2 or at vic 1 and vic 2. These results are similar to previous in vitro results. This field study also yielded seven heterokaryons between the paired homoallelic test strains and 32 heterokaryons between canker inciting strains and wild-type strains. None of the heterokaryons were infected with either hypovirus. In virulent cankers that acquired hypovirus, infection did not affect canker length or width. Likewise, sexual compatibility of the paired test strains did not affect perithecia and stromata production of the canker inciting strains. This study confirms that vic 1 and vic 2 have a strong influence over hypovirus transmission in a field setting, therefore future attempts at biological control of chestnut blight must consider the diversity of genes in Cryphonectria parasitica populations at sites where hypoviruses are released.

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