Isolation of Phytophthora palmivora Pathogenic to Citrus in Florida. S. E. Zitko, Senior Biologist, Citrus Research and Education Center, University of Florida, IFAS, 700 Experiment Station Road, Lake Alfred 33850. L. W. Timmer, and H. A. Sandler. Professor, and Former Senior Biologist, Citrus Research and Education Center, University of Florida, IFAS, 700 Experiment Station Road, Lake Alfred 33850. Plant Dis. 75:532-535. Accepted for publication 7 January 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0532.

A Phytophthora sp. recovered from soil in a citrus orchard near Fort Pierce, Florida, was characterized and designated as P. palmivora. It produced papillate, caducous sporangia (50 × 27 µm) with a pedicel 3 µm long. All isolates were A¹ and mated readily with A² mating types of P. palmivora from cacao and P. parasitica from citrus. Oogonia, oospores, and chlamydospores averaged 30, 25, and 34 µm, respectively, in diameter. The optimum temperature for growth was 30 C, with little or no growth at 15 or 33 C. The citrus isolate of P. palmivora was as pathogenic as or more so than P. parasitica to fibrous roots of sweet orange, sour orange, and Swingle citrumelo. A P. arecae isolate from palm that was morphologically similar to the citrus isolate was not pathogenic to citrus roots. The P. palmivora-citrus, P. arecae-palm, and P. parasitica isolates produced stem lesions on cuttings of citron cv. Etrog and on sweet orange seedlings. The citrus isolate of P. palmivora was as pathogenic as P. citrophthora to citrus fruit, and P. arecae from palm was also pathogenic to fruit. This is the first report of P. palmivora pathogenic to citrus in the United States.