Taxonomy and biology of the plant pathogenic fungus Thielaviopsis basicola

Abstract

The Ascomycete species Thielaviopsis basicola is a well-known pathogen of multiple important crop and ornamental plant species. This pathogen has been known for more than 150 years and has been extensively studied during this time, but important questions surrounding its taxonomy and mating behaviour remained to be answered. The first aim of this project was to resolve the taxonomic placement of the species using a multi-gene phylogenetics approach. Our research to address this aim revealed that the species represented a lineage distinct from all other genera in the Ceratocystidaceae, that we proceeded to describe as the new genus Berkeleyomyces. Our phylogenetic analyses also separated the collection of T. basicola isolates into two well-supported lineages within the genus, that we recognized as two distinct species. The first of these represented T. basicola, which was provided with a new combination, namely B. basicola. The second was described as a new species, named B. rouxiae. From the range of genera in which T. basicola was treated over the years, the name Milowia was technically available to accommodate the two species. However, we submitted a formal proposal to reject the name of the type species of this genus, M. nivea, and all names for which it serves as basionym, due to the absence of a type specimen and its dubious identity based on varying descriptions and illustrations by the same author across different publications. The second aim of our study was to determine the sexuality of these species, as some researches had suggested that T. basicola might exist exclusively asexually. To address this aim, we sequenced and assembled the whole genome sequence of the reference specimen of Berkeleyomyces basicola, designated when we described the genus. Using information from this genome we were able to determine the mating strategy of both species in the genus. Our results showed that both B. basicola and B. rouxiae contain all the typical genes required for heterothallic mating, but despite our efforts to obtain the sexual state in laboratory crosses, the sexual state of both species remains unknown. As a whole, the research conducted as part of this thesis has contributed to the global knowledge of these important pathogens by clarifying and settling the confusing taxonomy of the species, and by elucidating the mating strategy of these fungi. This will no doubt assist in improved diagnosis of the disease and a better understanding of the risks posed by introduced populations of these pathogens.