Hermetothecium mikaniae-micranthae T.F. Nóbrega, B.W. Ferreira, H.C. Evans & R.W. Barreto, sp. nov.

MycoBank number: MB 832760; Index Fungorum number: IF 832760; Facesoffungi number: FoF;

Etymology. Name reflects the host, Mikania micrantha.

Colonies hypophyllous, forming irregular white patches on the host surfaces, powdery mildew-like. Mycelium superficial, com- posed of very narrow (1– 4 μm diam), branched, almost indistinguishably septate, thin-walled, hyaline, hyphae, forming a dense colourless subiculum. Ascomata chasmothecium (similar to fruit bodies of the Erysiphales), scattered to gregarious, globose, 51–74 × 55 –76 μm diam, without an ostiole, walls thick- ened composed of 3 – 4 layers of brown textura globulosa, 7–17 μm, smooth. Asci fasciculate, subglobose, 19 – 27 × 7–12 μm, bitunicate, 8-spored. Sterile filaments absent. Ascospores el- lipsoid, 7–13 × 2 – 5 μm, 0 – 2-septate, hyaline, smooth. Asexual morph absent.

Typus. BRAZIL, Minas Gerais, Viçosa, campus of the Universidade Federal de Viçosa, coffee experimental area (Viveiro de Café), on living leaves of Mikania micrantha (Asteraceae), 4 Dec. 2018, R.W. Barreto (holotype VIC 47212, ITS and LSU sequences GenBank MN537723 and MN537725, MycoBank MB832760).

Notes — Numerous attempts to isolate this fungus on a range of general-purpose culture media failed to produce any culture, leading to the conclusion that this is a biotrophic taxon. Furthermore, detailed observations under the compound microscope and via scanning electron microscopy failed to produce any evidence of appressoria, or other penetration structures of leaf tissue, or any internal growth of mycelium. It appears that H. mikaniae-micranthae is an epiphyte relying strictly on plant exudates for its growth and is a specialised colonist of this plant host. Mikania micrantha is a relatively uncommon but widespread ruderal climber in Brazil, frequently associated with marshy areas. However, in its exotic range in the Palaeotropics, especially in Asia, it is highly invasive and damaging (mile- a-minute weed) in both natural and agricultural ecosystems (Ellison & Sankaran 2017).

Phylogenetic trees constructed from the analysis of Maximum Parsimony and Bayesian Inference demonstrated that the fungus belongs to the Chaetothyriaceae. Many species included in this family are epiphytes, colonising the surface of living leaves with mycelium limited to the host cuticle (Chomnunti et al. 2012). Sequences of the fungus, obtained directly from colonies on living leaves of M. micrantha, formed a clade isolated from other genera of Chaetothyriacae, with high support (bootstrap = 100 / posterior probability = 1) justifying the recognition of a new monotypic genus for this species.

The closest genera to Hermetothecium in the phylogenetic study are Phaeosaccardinula and Vonarxia. Fungi in Phaeosaccardinula have ascomata, with a dark, non-setose pellicle, saccate, bitunicate asci and muriform, hyaline to brownish ascospores (Yang et al. 2014). Vonarxia is based on an asexual morph which is sporodochial, with septate setae (Batista et al. 1960).