Pythiaceous fungi and plant-parasitic nematodes in California pear orchards: I. Occurrence and pathogenicity of pythiaceous fungi in orchard soils

Abstract

Pythiaceous Fungi and Plant-Parasitic Nematodes in California Pear Orchards I. During a year’s survey of 126 California pear orchards with varying degrees of pear-decline damage, a total of 3,586 isolates of soil-borne fungi of the genera Phytophthora and Pythium were recovered, and, where possible, identified as to species. A direct relationship was found between the severity of pear-decline damage in an orchard and the percentage of isolates from the orchard that were Pythium spp.; and a corresponding inverse relationship between pear-decline damage severity and the percentage of isolates that were Phytophthora spp. Isolates of Phytophthora cactorum and P. cryptogea were found to be pathogenic on Pyrus communis L. hort. var. Winter Nelis and Pyrus serotina Rehd. seedlings. These pathogens were found infrequently, and it was concluded that pythiaceous fungi were not the primary cause of pear decline. Pear fruit and pear and apple seedlings trapped more Pythium spp. than the fruit of apple, lemon, and avocado, while few Phytophthora spp. were trapped in seedlings. Phytophthora spp. isolates were trapped most often in pear and apple fruit. Lemon and avocado fruit yielded a fair number of both genera.