Abstract
In Europe as in North America, elms are devastated by Dutch elm disease (DED), caused by the alien ascomycete Ophiostoma novo-ulmi. Pathogen dispersal and transmission are ensured by local species of bark beetles, which established a novel association with the fungus. Elm bark beetles also transport the Geosmithia fungi genus that is found in scolytids’ galleries colonized by O. novo-ulmi. Widespread horizontal gene transfer between O. novo-ulmi and Geosmithia was recently observed. In order to define the relation between these two fungi in the DED pathosystem, O. novo-ulmi and Geosmithia species from elm, including a GFP-tagged strain, were grown in dual culture and mycelial interactions were observed by light and fluorescence microscopy. Growth and sporulation of O. novo-ulmi in the absence or presence of Geosmithia were compared. The impact of Geosmithia on DED severity was tested in vivo by co-inoculating Geosmithia and O. novo-ulmi in elms. A close and stable relation was observed between the two fungi, which may be classified as mycoparasitism by Geosmithia on O. novo-ulmi. These results prove the existence of a new component in the complex of organisms involved in DED, which might be capable of reducing the disease impact.
Highlights
Every species is intricately involved with a myriad of associates—some obligate, some facultative—that profoundly influence their evolution, physiology, and life history [1].Dutch elm disease (DED) is a highly destructive vascular disease, which caused an extensive loss of mature elms in Europe, Asia, and North America during the twentieth century
The growth rate of Ophiostoma novoulmi Brasier (ONU) strains was generally higher in dual culture with Geosmithia spp. isolated from elm than in pure culture, both on MEA and CZD (Fig. 1)
The reactions observed between the mycelia of Ophiostomatoid fungi and Geosmithia species were here classified into five main types, ranging from fully intermingling colonies to mutual growth inhibition (Table 2, Fig. 2): Type 1, fully intermingling: complete equal bidirectional mycelial penetration
Summary
Dutch elm disease (DED) is a highly destructive vascular disease, which caused an extensive loss of mature elms in Europe, Asia, and North America during the twentieth century. Pathogen spreading and infection of suitable hosts are mainly ensured by elm bark beetles (EBB) (Coleoptera: Curculionidae, Scolytinae) [3]. The synchrony between the life cycles of host tree, fungus, and EBB allows vectors to disseminate ONU when host plants are most prone to infection and temperatures favorable for fungal growth, boosting the pathogen’s aggressiveness [4]. Mites may contribute to DED transmission by spreading the fungus within the gallery system, enhancing ONU sexual reproduction and promoting an increase in genetic diversity through the fertilization of protoperithecia. Mites may contribute to increase the spore load beyond the threshold required for infection [5]
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