The saprotrophic dimension of Exobasidium (Exobasidiales, Basidiomycota): evidence for greater diversity and ecological flexibility than previously recognized

Abstract

Fungi represent the largest group of plant pathogens, causing significant economic losses in agriculture and forestry worldwide. Species of the genus Exobasidium (Exobasidiales, Basidiomycota) are considered pathogens of plants from the order Ericales. While Exobasidium species notably impact tea and fruit production, their complete life cycles remain poorly understood, which hampers their management.

These species are characterized by a dikaryotic parasitic stage tightly associated with living host tissues and a haploid, yeast-like saprotrophic stage. The prevalence, ecological significance, and potential contribution of this saprotrophic stage to the persistence of Exobasidium outside living host plants remain understudied. In this study, we confirmed the presence of several Exobasidium species in the leaf phyllosphere of five broad-leaved tree species in Central Europe using both cultivation and environmental DNA ITS 2 - based approaches.

Additionally, we describe a new species without a known parasitic phase, E. phylloplanum sp. nov., along with its physiological description and annotated genome. Environmental DNA surveys, using the GlobalFungi database, revealed that E. phylloplanum is not only common locally but also the most frequently detected Exobasidium lineage worldwide. This broad ecological amplitude contrasts with the narrow host specificity typical of parasitic congeners, suggesting that E. phylloplanum has adapted to a generalist saprotrophic life-history strategy.

Our findings demonstrate that Exobasidium species can persist saprotrophically on diverse hosts, suggesting a broader ecological role and higher diversity than previously recognized. This research elucidates the diverse ecological roles of Exobasidium species and suggests that further genomic studies could reveal the genetic factors that underpin the different life strategies within this genus.