Seasonal Differences in Susceptibility of Three Citrus Rootstocks to Root Lesions Caused by Phytophthora citrophthora and P. parasitica. M. E. Matheron, Extension Plant Pathologist, University of Arizona, Yuma Agricultural Center, Yuma 85364. J. C. Matejka, Research Specialist, University of Arizona, Yuma Agricultural Center, Yuma 85364. Plant Dis. 77:729-732. Accepted for publication 5 April 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/PD-77-0729.

Root and shoot segments from Lisbon lemon trees established on sour orange, rough lemon, and volkamer lemon rootstocks were evaluated for seasonal changes in susceptibility to Phytophthora citrophthora and P. parasitica. Plant material was wounded, inoculated with P. citrophthora or P. parasitica, then incubated in moist chambers. Root segments of sour orange, rough lemon, and volkamer lemon inoculated with P. citrophthora developed shorter lesions during January–February than during July–October. Similarly, lesions on excised root tissue of sour orange inoculated with P. parasitica were shorter during January–February than during July–December. The mean annual lesion size on root pieces inoculated with P. citrophthora or P. parasitica was significantly greater on sour orange than on rough lemon and volkamer lemon. In contrast, the mean annual lesion size on shoot pieces of Lisbon lemon collected from trees established on sour orange, rough lemon, and volkamer lemon rootstocks and inoculated with the same pathogens did not differ. A significant linear relationship was observed between the number of days in December and January with a mean air temperature below 10 C and the length of lesions that developed during January–February on root segments of sour orange inoculated with either pathogen. Seasonal changes in susceptibility of citrus rootstocks to P. citrophthora and P. parasitica may facilitate timing of disease control measures to coincide with periods when disease development is greatest.