Studies on graminicolous species of Phyllachora Fckl. IV. Evaluation of the criteria of species

Abstract

Recognition of the large number of graminicolous Phyllachora spp, by various authors is attributed to the use of unreliable morphological characters as taxonomic criteria, as well as acceptance of the concept of each Phyllachora species being hostspecific. It is considered that in fact there are far fewer Phyllachora spp. than reported in the literature and an attempt is made to clarify the position by making a study of Phyllachoras which occur on grasses.

By considering in turn each character previously used to delimit species and by studying the degree of variation of each character within individual specimens as well as groups of specimens on closely related hosts, and between groups of specimens on unrelated hosts, it is possible to evaluate the usefulness of each character in taxonomy.

It is shown that of all the characters previously used only two are sufficiently stable for taxonomic purposes. These are the length of the ascus pedicel relative to that of the ascosporific portion of the ascus, and the morphology of ascospores. Ascospore morphology is particularly useful, but only when all the various shapes that can occur in individual specimens are noted and when ascospore shape, as distinct from outline, is used.

The use of certain other characters of Phyllachoras is proposed. These are: appressorium morphology (when appressoria are produced from ascospores germinated on host tissue) and the morphology of the sporophores produced by the Leptostromella states of certain Phyllachoras. It is suggested that characters of imperfect states of Phyllachora will also provide useful taxonomic characters when these states are known to exist in more species than at present.

As well as the characters listed as reliable for delimiting species, other characters such as ascospore size and arrangement, ascus length, and sporophore size may have limited usefulness when used to confirm a diagnosis, but not to delimit species.