Abstract A description is provided for Aphanomyces raphani . Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. HOSTS: Species in the Cruciferae only. Brassica alboglabra, B. carnet, B. campestris var. pekinensis, B. chinensis, B. juncea, B. napobrassica, B. napus, B. nigra, B. oleracea (vars acephala, botrytis, capitata, gemmifera, gongylodes, italica, viridis), B. perviridis, B. rapa, B. robertiana, Crambe abyssinica, Eruca saliva, Lunaria annua , Raphanus raphanistrum, R. sativus, Sinapis alba . DISEASE: Black-root disease of radish; the fungus is a facultatively necrotrophic plant pathogen. Small black, water-soaked lesions on the lower hypocotyl and epidermis accompanied by slight longitudinal cracking of the subepidermal tissues become visible 20 days after sowing. In seedlings, the roots, hypocotyls, petioles and cotyledons become blackened and collapse. On older plants horizontal spread of the fungus produces characteristic constrictions and irregular bands around the circumference of the root (girdle disease). Subsequent penetration of the inner cortex results in blackening of the internal tissues in a portion or all of the root, accompanied by deformity of the roots and stunting of the tops due to retardation of growth. Lesions become invaded rapidly by secondary colonisers such as soft-rot bacteria. GEOGRAPHICAL DISTRIBUTION: Africa: South Africa. Asia: Japan. Australasia & Oceania: Australia (NSW Qld), New Zealand. Europe: Germany, Rumania. North America: Canada (British Columbia, NS, Ont, Que), USA (California, IL IA, IN, ME to Florida, OR). South America: Venezuela. See CMI Distribution Maps of Plant Diseases 421. TRANSMISSION: In wet soil, by zoospores which accumulate at root tips in response to hypocotyl exudates or indole-3-aldehyde (59, 3937). The large number of crucifers (particularly cole crops) infected by A. raphani allows it to persist even in the absence of a radish crop. Oospores are generally believed to survive in soil, although there is no evidence to support this. The oospore germinates via a hypha which produces an apical sporangium.