Two Problematical Unrecorded Fungal Diseases of British Grasses

Abstract

Summary. The difficulties of naming and recording two parasites recently found in British grasses are discussed. Attempts to find accceptable names for two recently observed parasites of British grasses illustrate the taxonomic and nomenclatorial problems posed by parasitic microfungi with very few distinctive morphological characters. In the writer's lifetime the pendulum of fashion in this matter has swung from one extreme to the other. Formerly a new specific name was coined for every freshly recognised biological race of a parasitic fungus, especially in Uredinales and Ustilaginales. Practical problems in recognising such taxa in herbarium material led to a convention that species of these fungi shall be distinguished solely on morphological criteria. With Uredinales the consequences of the changed attitude seem generally acceptable, especially as very fine morphological differences have been held to justify specific rank for taxa. For example, separation of Puccinia schismi Bub. from P. recondita Rob. & Desm. rests on a difference of up to 1 m in thickness of the teleutospore wall (Wilson & Henderson 1966), though there are supporting biological criteria. Among Ustilaginales the consequences of a rigid morphological approach to speciation have been less happy. A farmer's unaided eye can readily distinguish between the covered and loose smuts of wheat and barley, important distinctions because the fungi involved have different life histories and require very different control measures. Yet because the microscope can detect only minute or inconstant differences in their spores and those of similar diseases on oats the fungi in question must be grouped as at best varieties of a single species Ustilago segetum (Pers.) Rouss. A cynic can afford to smile at the virtual disregard of classic experimental work in disentangling the cereal smuts: farmers and plant pathologists are more likely to swear, but the taxonomic problem is real. Fortunately the parasitic fungi described below are unlikely to prove of economic importance so no ill consequences are to be feared from taxonomic or nomenclatural antics concerning them. To the unaided eye their sori appear very similar and may be confused also with covered teleutosori of a rust but they belong to widely separated fungi. The first is a species of Physoderma, a genus currently referred to the Blastocladiales. Leafspots dark grey, slightly swollen, 1-5-2 x up to 1 mm; resting spores usually two or more within a cell of the host mesophyll, subglobose, shortly ellipsoid or slightly irregular in outline by mutual pressure, 23-38 x 18-28 g.m with smooth brown wall 1-1-5 gm thick and with a large central guttule.