3.5.7

TRACKING THE DISSEMINATION OF GENETICALLY CHARACTERIZED HYPOVIRUS-INFECTED CRYPHONECTRIA PARASITICA STRAINS AFTER A BIOCONTROL TREATMENT


PJ HOEGGER 1, C ALLEMANN 1, O HOLDENRIEDER 2, D RIGLING 1 and U HEINIGER 1

1 Swiss Federal Institute for Forest, Snow and Landscape Research, CH-8903 Birmensdorf, Switzerland. 2 Section of Forest Pathology and Dendrology, Swiss Federal Institute of Technology, CH-8092 Zurich, Switzerland.

Background and Objectives
Cryphonectria hypoviruses are of great interest for biological control of chestnut blight. However, the exact mechanisms of their dissemination are unclear. Sporulation of hypovirulent strains is reduced and the hypovirus is only transmitted into asexual pycnospores but not into ascospores. In addition, the system of vegetative incompatibility in C. parasitica limits the transmission of the virus between fungal strains. Nevertheless, the hypoviruses spread very efficiently in European populations [1]. For a better understanding of their dissemination, a hypovirus has been artificially introduced into two different C. parasitica populations by treating chestnut blight cankers with a hypovirus-infected strain. In one population (Chox, VS) hypovirulence was absent before the treatment, whereas in the other population (Claro, TI) it was already naturally present. The aim of the experiment is to track the introduced virus as well as its fungal carrier and to survey the dynamic of the disease in these stands.

Material and Methods
In Chox the virus is easily detected by verifying the virus-associated culture morphology of the isolated strains as there are no other viruses present. In Claro, RFLPs of two fragments (one in each open reading frame) amplified from the reverse transcribed viral genome (RT-PCR) serve as markers to distinguish the introduced virus from the resident hypoviruses. Similarly, fingerprinting methods are used to identify the introduced fungal strain and its mitochondria [2]. With these markers, the transmission and dissemination of the three genomes (nuclear, mitochondrial and viral) of the introduced hypovirus-infected strain is followed.

Results and Conclusions
In Chox 25% of the cankers were treated in fall 96. In Claro, about 20% were treated. Together with the naturally occuring hypovirulence this resulted in a total of 64% hypovirus-infected cankers in Claro. In fall 97, 22% of the untreated, hypovirus-free cankers in Chox and 50% in Claro were found to be converted. Furthermore, 22% of the newly developed cankers in Chox and 60% in Claro yielded hypovirus-infected isolates. Already one year after the treatment hypovirulence is well established in the Chox plot. The higher rates of hypovirus infection among the existing and the newly developed cankers in the Claro plot can be explained by the presence of the natural hypovirulent inoculum.

Preliminary results indicate that the introduced hypovirus is efficiently transmitted into the treated cankers and from there it is spread within the plots. Furthermore, we have evidence for mitochondrial transmission and dissemination but at much lower frequencies compared to the viruses. The introduced fungal carrier seems not to be disseminated during the observation period.

References
1. Heiniger U, Rigling D, 1994. Annual Review of Phytopathology 32, 581-599.
2. Milgroom MG, Lipari SE, 1993. Phytopathology 83, 563-567.