A description is provided for Phytophthora nicotianae var. parasitica. Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. HOSTS: On a very wide range of host plants comprising 58 families including: avocado, castor, Cinchona spp., citrus, cotton, eggplant, guava, lucerne, papaw, parsley, pineapple, Piper betle, rhubarb, sesame, strawberry, tomato. DISEASES: Damping-off of seedlings (tomato, castor, citrus, cotton); root rot (citrus, avocado, strawberry, lucerne); crown rot (parsley, rhubarb, strawberry, lucerne); brown stem rot of tobacco; stem canker and tip blight of Cinchona spp. ; leaf blight (castor, sesame, pineapple, Piper betle) and fruit rot (citrus, tomato, guava, papaw, eggplant). GEOGRAPHICAL DISTRIBUTION: Africa (Ethiopia, Mali, Madagascar, Mauritius, Morocco, Nigeria, Sierra Leone, Southern Rhodesia, Tanganyika); Asia (Burma, Ceylon, China, Formosa, India, Israel, Japan, Java, Malaya, Philippines); Australia & Oceania (Australia, Hawaii, Tasmania); Europe (Cyprus, France, Germany, Great Britain, Holland, Ireland, Italy, Poland, Portugal, U.S.S.R.); North America (Bermuda, Canada, Mexico, U.S.A.); Central America & West Indies (Costa Rica, Cuba, El Salvador, Guatemala, Jamaica, Montserrat, Puerto Rico, Trinidad);. South America (Argentina, Brazil, British Guiana, Colombia, Paraguay, Peru, Venezuela). TRANSMISSION: Soil-borne, spreading rapidly after heavy rain or where soil remains moist or water-logged (40: 470). Also recorded in drainage water in India and in reservoirs and canals supplying citrus groves in U.S.A. (23: 45; 39: 24). A method for determining a disease potential index in soil using lemon fruit has been described (38: 4). Also present in testas of seeds from diseased citrus fruit which may infect nursery seedbeds (37: 165).