The use of transmissible hypovirulence for the biological control of Ophiostoma, the causal agent of Dutch elm disease

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Natural and urban populations of the American elm have been devastated by pathogenic fungi of the genus Ophiostoma, the causal agent of Dutch elm disease (DED). Control of this pathogen has focused on fungicides to stem the growth of the fungus or pesticides to control the spread of the insect vector however neither option has been very effective. An attractive alternative to the use of chemical pesticides or fungicides is the development of a biological control for O. novo-ulmi. This requires, at minimum, an agent that is antagonistic to the fungus, is transmissible to extant populations of the fungus in the field, and is very specific to minimize off-target effects. Hypovirulence is the decreased virulence of a pathogen towards its host and can result from many causes including mitochondrial DNA mutations, nuclear genome mutations or the presence of mycoviruses. We intend to extend the application of transmissible hypovirulence for the management of fungal pathogens by the development of a transmissible disease control using engineered dsRNA vectors for the biological control of DED. Induction of hypovirulence via mycoviruses could readily be applied to O. novo-ulmi as the target population at the western Canadian disease front is essentially composed of two very large clones and vegetative compatibility barriers are rare. We have determined the genome sequence of a new dsRNA mitovirus OnuMV1c isolated from an infected O. novo-ulmi at disease front and demonstrated its transmission between strains. Partitiviruses, which reside in the cytoplasm, have been described in closely related species and are expected to be transmissible to O. novo-ulmi. Through our work on the genomics of O. novo-ulmi we have identified several gene loci thought to be integral to pathogenicity and fungal fitness. We intend to embark on the re-engineering of viral dsRNAs by the insertion of silencing RNA (siRNA) specific to identified O. novo-ulmi gene loci to selectively down-regulate genes critical to fungal survival. The development of RNAi engineered hypoviruses could provide a new approach for biological control that would be applicable to many fungal pathogens.