Duplex real-time PCR assay for the simultaneous detection of Ophiostoma novo-ulmi and Geosmithia spp. in elm wood and insect vectors

Dutch elm disease (DED) is a destructive tracheomycosis caused by Ophiostoma novo-ulmi, an ascomycete probably originating in East-Asia that is devastating natural elm populations throughout Europe, North America and Asia. The fungus is mainly spread by elm bark beetles that complete their life cycle between healthy and diseased elms. Recently, it has been highlighted that some fungi of the genus Geosmithia, which are similarly well associated with bark beetles, seem to also play a role in the DED pathosystem acting as mycoparasites of O. novo-ulmi. Although some relationship between the fungi is clear, the biological cycle of Geosmithia spp. within the DED cycle is still partly unclear, as is the role of Geosmithia spp. in association with the bark beetles. In this work, we tried to clarify these aspects by developing a qPCR duplex TaqMan assay to detect and quantify DNA of both fungi. The assay is extremely sensitive showing a limit of detection as low as 2 fg μl–1 for both fungi. We collected woody samples from healthy and infected elm trees throughout the beetle life cycle. All healthy elm samples were negative for both Geosmithia spp. and O. novo-ulmi DNA. Geosmithia spp. are never present in infected, but living trees, while they are present in frass of elm bark beetles (EBB – Scolytus spp.) and at each stage of the EBB life cycle in much higher quantities than O. novo-ulmi. This work provides a better understanding of the role and interactions occurring amongst the main players of the DED pathosystem.