Exobasidiomycetes

Exobasidiomycetes is a small, poorly understood order of fungi within the phylum Basidiomycota. Historically, this group was considered a distinct order, but modern phylogenetic studies have placed its members within other established orders. The defining characteristic of the Exobasidiomycetes, as a traditional grouping, was their unique mode of reproduction, where basidia form directly on the surface of host plant tissues, often on leaves and stems, causing hypertrophy and malformations. These fungi are obligate parasites, meaning they require a living host to survive.

The most well-known genera historically associated with Exobasidiomycetes include Exobasidium, which causes galls and leaf distortions on various Ericaceae plants such as blueberries, rhododendrons, and azaleas. Another genus, Endğrafla, also exhibits similar parasitic behaviors. The symptoms caused by these fungi typically involve swollen, distorted, and sometimes brightly colored growths on the infected plant parts. The fungus forms a layer of basidia and basidiospores on the hypertrophied surface, which are then dispersed by wind.

Despite their economic and ecological impact on host plants, research on Exobasidiomycetes has been limited. Their placement within fungal systematics has been a subject of ongoing debate, with molecular data suggesting that the traditional order may be polyphyletic, meaning it does not include all descendants of a common ancestor. Consequently, individual genera and species previously assigned to Exobasidiomycetes are now being reclassified into more appropriate taxonomic groups based on genetic evidence. This ongoing reevaluation reflects the dynamic nature of fungal classification.