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Twenty species of lichenicolous fungi are reported onThamnoliaspecies and discussed.Epithamnolia karatyginiigen. et sp. nov.,Capronia thamnoliaesp. nov.,Cercidospora epithamnoliasp. nov.,C. thamnogalloidessp. nov.,C. thamnoliaesp. nov., andSphaerellothecium thamnoliaesp. nov. (var.thamnoliae, var.taimyricum) are described fromThamnolia.Dacampia thamnoliicolaandPhoma thamnoliaeare introducedad interim.Cercidospora lecidomaeis reduced to synonymy withC. punctillata.Polycoccum vermiculariumis new to Asia,Odontotrema santessoniiandO. thamnoliaeare new to North America,Cladosporium licheniphilumis new to the Arctic,Thamnogalla crombieiis new to Greenland and Svalbard,Stigmidium frigidumis new to Mongolia and confirmed in the USA,Lichenopeltella thamnoliaeis new to Bolivia.Cladosporium licheniphilumandPhaeospora arcticaare newly documented onThamnoliaandLichenopeltella thamnoliaeonThamnolia papelillovar.subsolida.Thamnolia vermicularis, supporting 23 species of lichenicolous fungi, is shown to be the 15th most hospitable lichen species in the world. A worldwide key to 23 species of fungi known to occur onThamnoliais provided.

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Nomenclatural type definitions are one of the most important concepts in biological nomenclature. Being physical objects that can be re-studied by other researchers, types permanently link taxonomy (an artificial agreement to classify biological diversity) with nomenclature (an artificial agreement to name biological diversity). Two proposals to amend the International Code of Nomenclature for algae, fungi, and plants (ICN), allowing DNA sequences alone (of any region and extent) to serve as types of taxon names for voucherless fungi (mainly putative taxa from environmental DNA sequences), have been submitted to be voted on at the 11th International Mycological Congress (Puerto Rico, July 2018). We consider various genetic processes affecting the distribution of alleles among taxa and find that alleles may not consistently and uniquely represent the species within which they are contained. Should the proposals be accepted, the meaning of nomenclatural types would change in a fundamental way from physical objects as sources of data to the data themselves. Such changes are conducive to irreproducible science, the potential typification on artefactual data, and massive creation of names with low information content, ultimately causing nomenclatural instability and unnecessary work for future researchers that would stall future explorations of fungal diversity. We conclude that the acceptance of DNA sequences alone as types of names of taxa, under the terms used in the current proposals, is unnecessary and would not solve the problem of naming putative taxa known only from DNA sequences in a scientifically defensible way. As an alternative, we highlight the use of formulas for naming putative taxa (candidate taxa) that do not require any modification of the ICN.