Role of yeast-mycelium transition in the pathogenicity of the Dutch elm disease fungus Ophiostoma novo-ulmi

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Dutch elm disease (DED) is the most destructive disease of elms. It has been estimated that, during the last 100 years, over one billion mature elms were killed by two successive pandemics of DED caused by two microscopic fungi called Ophiostoma ulmi and O. novo-ulmi. Our ongoing research suggests that an Ophiostoma gene (called Omp2) coding for a high affinity ammonium transporter is important for the virulence of the DED pathogens towards elms. We intend to clarify the role of Omp2 in pathogenesis. In particular, we will verify the hypothesis that the action of Omp2 stems from its effect on the regulation of yeast and mycelium forms of the DED fungi in the presence of low concentrations of ammonium. We further propose to analyse two additional genes which may be involved in other regulatory pathways for fungal growth, as suggested by the recovery of O. novo-ulmi mutants with impaired yeast or mycelial development. The short-term benefit of this project is a better understanding of how the DED fungi develop in elms and eventually kill them. Anticipated long-term benefits are the identification of fungal targets for the development of efficient and environmentally acceptable control methods to save the elms.