Didymella festucae and its imperfect state, Phleospora idahoensis, on Festuca species in western North America

Abstract

Didymella festucae (Weg.) Holm (imperfect state Phleospora idahoensis Sprague), endemic on native Festuca spp. in western North America and reported from a few arctic–alpine regions in Europe, is the cause of a major destructive stem eyespot disease in seed crops of red fescue, Festuca rubra L., in the Peace River region of northern Alberta and British Columbia. The original description of the disease as a leaf and pedicel blight requires modification because eyespots, staining, and later extensive blackening of culms and inflorescences are the common symptoms. The size range of ascospores, 18–63 × 4.5–14.0 microns (μ), and conidia, 18–105 × 3.0–9.0 μ, considerably exceeded dimensions previously given for D. festucae from Europe and P. idahoensis from North America and Europe. Although the morphology of fungal structures was highly variable, dimensions of ascospores and conidia from the Swiss type material fell into the size range established for North American material. Swiss type material of the perfect stage also yielded typical conidia of P. idahoensis. Microconidia are uncommon in natural material and cultures and their function is uncertain. Pathogenicity tests confirmed the wide natural-host range. Host immunity or physiologic specialization of the fungus is not indicated, but there is some evidence that isolates differ in degree of pathogenicity. For the present, P. idahoensis is suitably accommodated in Phleospora and D. festucae in Didymella, although both genera are ill-defined, requiring further study.