Spiroplana centripeta gen. & sp. nov., a leaf parasite of Philadelphus and Deutzia with a remarkable aeroaquatic conidium morphology

Spiroplana centripeta is described as a new genus and species from Korea. Its spirally coiled conidia closely resemble those of the aeroaquatic genus Spirosphaera but its ecology differs totally, as it is parasitic to living Philadelphus and Deutzia leaves causing symptoms superficially similar to powdery mildew disease. No sexual state has been found, but molecular phylogenies inferred from ITS1-5.8S-ITS2 rDNA and partial nuLSU sequences support it within the Pleosporales (Dothideomycetes) and thus phylogenetically distinct from the generic type, Spirosphaera floriformis in the Leotiomycetes. Molecular phylogenies further show that Spirosphaera is polyphyletic and that generic diagnostic characters have evolved multiple times as an adaptation to conidium dispersal in the aeroaquatic niche. Morphologically, Spiroplana centripeta differs from Spirosphaera in its branching pattern, characterised by a main coil of cells in the conidial filament that give rise to 1-2 daughter filaments only on the inner side of the main coil. The daughter filaments then grow, coiling inwards with occasional additional branching to produce a tightly interwoven propagule enclosing air in a manner similar to aeroaquatic fungi. As primary branching takes place in one plane, the conidia are laterally flattened. In light of these molecular, morphological and ecological differences, a new genus is described. We believe the astounding similarity of the Spiroplana and Spirosphaera conidia is related to dispersal on the surface of a water film.