Effects of Temperature on Sporulation and Growth of Phytophthora citrophthora and P. parasitica and Development of Foot and Root Rot on Citrus. M. E. Matheron, Extension Plant Pathologist, University of Arizona, Yuma Agricultural Center, Yuma, AZ 85364. J. C. Matejka, Research Specialist, University of Arizona, Yuma Agricultural Center, Yuma, AZ 85364. Plant Dis. 76:1103-1109. Accepted for publication 10 July 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-1103.

Abundant production of sporangia by Phytophthora citrophthora and P. parasitica occurred between 20 and 30 C. P. citrophthora was recovered from at least 50% of tested rootlets of Citrus jambhiri (rough lemon) plants incubated at temperatures of 10, 15, 20, and 25 C in the presence of preformed sporangia, whereas P. parasitica was recovered from at least 50% of tested rootlets at 15, 20, 25, and 30 C. Lowest recovery of either pathogen from rootlets of rough lemon occurred after incubation at 10 and 35 C for P. parasitica and 35 C for P. citrophthora. Maximum recovery from rootlets of rough lemon seedlings in the presence of soil naturally infested with P. citrophthora and P. parasitica was recorded at temperature ranges of 15–25 C and 15–30 C, respectively. Greatest growth of P. citrophthora on cornmeal agar (CMA) occurred at 25 C, whereas maximum development of lesions on stems of rough lemon or excised root pieces of C. aurantium (sour orange) inoculated with this pathogen was recorded at 10–20 or 20–25 C, respectively. Radial growth of P. parasitica on CMA was greatest at 30 C, whereas the largest lesions on stems of rough lemon or excised root segments of sour orange inoculated with this pathogen were recorded at 15–25 or 20–25 C, respectively. Soil temperatures that were recorded during certain times of the year in a citrus orchard in Yuma, Arizona, were found to be inhibitory to sporulation and growth of P. citrophthora and P. parasitica as well as development of lesions on citrus stem and root tissues in the laboratory.