Biological control of stem canker fungi

A model system for studying dsRNA mycoviruses in the genus Diaporthe has been developed. Research has revealed that all hypovirulent strains of D. ambigua under investigation harbour a 4 kb dsRNA virus. The complete sequence of the dsRNA element has been published. This Diaporthe ambigua RNA (DaRV) mycovirus does not show sequence similarity to any known mycovirus and does not appear to code for a coat protein, but is anchored on the membranes of the host fungus. Full-length cDNA copies of the genome of DaRV were constructed and the plasmids were used to synthesise in vitro, viral RNA. The in vitro-produced RNA was subsequently used in transfection studies of Phomopsis sp. and D. ambigua in order to study the potential of these viruses to be used as biological control agents. One isolate of Phomopsis sp. and three isolates of D. ambigua were successfully transfected with DaRV virus. This is only the second report worldwide of transfection using a mycovirus other than the well-known Cryphonectria parasitica hypovirus. Another important finding pertained to the identity of the fungus from which DaRV was first isolated. It was shown that this fungus is Diaporthe perjuncta and not D. ambigua as previously thought. The acronym DaRV should therefore rather be used for Diaporthe RNA virus. This virus was successfully transfected into two other species of Diaporthe, D. ambigua and a Phomopsis sp. for which the Diaporthe state has not yet been found. The studies in which the virus was transmitted by hyphal anastomosis gave different results from those obtained by transfection. Mycoviruses isolated from Leucostoma persoonii were not associated with hypovirulence and therefore not characterised.