This 1955 article by G. C. Ainsworth examines how host-parasite relationships influence fungal taxonomy. Ainsworth discusses two primary approaches: spatial relationships (where parasites localize on specific host tissues or organs) and host specialization (physiological preferences for particular hosts). While spatial relationships like leaf versus stem infection have traditionally differentiated fungal genera such as Phoma and Phyllosticta, Ainsworth argues this criterion is often merely convenient rather than biologically meaningful. Host specialization has more extensively shaped fungal classification, particularly at the species level, with many species names derived from host plant names. However, this creates problems: it artificially inflates species numbers in groups like rust fungi (Uredinales), where over 300 biological races exist for a single forma specialis. Ainsworth advocates for a fundamental shift in mycological taxonomy toward defining fungi by intrinsic mycological characteristics rather than host-related features. He argues fungi should be classified 'for what they are, not for what they do or where they occur,' supporting reclassification of host-specialized forms as physiologic races or synonyms when morphologically similar.

Key findings

• Spatial localization of parasites on host tissues and organs has been used to differentiate fungal taxa but often reflects practical convenience rather than biological reality
• Host specialization has led to excessive species proliferation in certain fungal groups, with rust fungi averaging 36 species per genus compared to 8.3 overall for Ascomycetes
• Biological races, particularly in heteroecious rusts with multiple alternate hosts, create taxonomic confusion when given species rank despite morphological indistinguishability
• Modern mycological practice should prioritize intrinsic mycological characteristics over host-based criteria for species definition and classification
• Host-parasite relationships have practical value but should not be the primary basis for fungal taxonomy at the species level