A Phytophthora boll rot disease of cotton, Gossypium hirsutum has been found in Kern County in the Southern San Joaquin Valley of California. This disease is distinctly different from the common boll diseases reported from that area. It has been found only in sprinkler irrigated fields. In contrast to boll diseases previously reported this pathogen attacks unopened bolls, even those that have no visible insect injury. On many plants, bolls from the bottom to close to the top of the plants representing from 1/2 to over 3/4 of the potential production of the plant were affected. Initially, affected bolls had a watery black appearance beginning at the tip and spreading very rapidly toward the base. As the disease developed a snowy white growth was apparent over the surface of the blackened portion. Microscopic examination revealed this white growth was largely composed of sporangia of the fungus Phytophthora. Subsequent inoculation studies at U.C. Davis and Shatter with pure cultures of this fungus have shown it capable of producing the symptoms observed on diseased plants in the fields. Inoculated bolls were thoroughly diseased in a few days when placed in a humid chamber at 22 -23 C. Pure cultures of the fungus were identified as a Phytophthora capsici Leonian. In an attempt to identify the source of the problem an irrigation holding pond at one of the field sites was baited for the presence of the pathogen with tomato and pear fruits. Efforts are being made to identify and check the pathogenicity of the Phytophthora species isolated from the baits. In one of the infested fields attempts have been made to control the disease with fungicide sprays.