Avocado root rot, caused by the fungus Phytophthora cinnamomi, is the most important disease affecting avocados throughout the world. In California approximately 4,000 acres of avocados have been damaged. Trees growing in many types of soils have been affected, but the amount or extent of damage is much greater on some soils than on others. Previous investigators have pointed out that when the fungus is present, avocado trees growing in soils with poor internal drainage are most susceptible to root rot damage. Recent surveys have shown that the rate of spread is usually slow on deep, medium-textured (loam, fine sandy loam) soils and will range from moderate to rapid on many other soils.

Farm Advisors J. Beutel, Los Angeles County; C. Delphey, Ventura County; G. Goodall, Santa Barbara County; D. Gustafson, San Diego County; J. La Rue, Tulare County; M. Miller, Riverside County; and R. Puffer, San Bernardino County, were especially helpful in this survey, as have been J. Miner, G. Borst, and D. Hanson of the Soil Conservation Service.