Using molecular markers to assess the establishment and spread of a mycovirus applied as a biological control agent against chestnut blight

Chestnut blight, caused by the invasive fungus Cryphonectria parasitica, is one of the few tree diseases in Europe for which biological control is possible. The fungus may be infected by a mycovirus that reduces its virulence and sporulation capacity. In this study, we used molecular markers to assess the establishment and dissemination of the Cryphonectria hypovirus (CHV1) after its release as a biological control agent. The study was conducted in two chestnut stands in northern Switzerland. In both stands, viruses were repeatedly introduced by conducting classical canker treatments. Sequence analysis of viral isolates revealed the presence of the released viruses in the majority of the treated cankers, confirming that therapeutic canker treatment is mostly effective. Analysis of the dissemination of the released viruses gave contradictory results. In both stands, more than 50 % of the sampled untreated cankers were found to be virus-infected. In one stand, all viral isolates from those cankers originated from the released viruses, which evidently had spontaneously spread in the local C. parasitica population. In the other stand, the released virus was outcompeted by a naturally occurring virus, which was dominant in untreated cankers. Our study emphasizes the usefulness of molecular markers to track biological control agents and to evaluate the success of their application.