Comparative studies of Phoma spp. associated with potato gangrene in Northern Ireland

Abstract

Cultural, morphological, biochemical, physiological and pathogenic differences between Phoma exigua Desm. ( P. solanicola Prill. & Delacr.), P. exigua Desm. var. foveata (Foister) Boerema ( P. foveata Foister) and P. eupyrena Sacc. are described and discussed. P. eupyrena was distinguishable by morphological and biochemical characteristics from the other two fungi and was non-pathogenic to potato tubers and growing potato stems. P. exigua and its var. foveata were morphologically indistinguishable and both were pathogenic to potato tubers and stems when inoculated into wounded tissue. They differed culturally on malt agar, particularly when freshly isolated, physiologically in their maximal and optimal temperatures for spore germination and growth in culture, biochemically in the ability to produce diffusible anthraquinone pigments, and in their degree of pathogenicity to unwounded potato tubers. It is considered that because of their common characteristics these two fungi can be assigned to one species, the ubiquitous soil-borne fungus P. exigua , but that their differences are sufficient to regard the gangrene fungus ( foveata ) as a distinct variety.