Relative Resistance of Different Rootstocks of English Walnut to Six Phytophthora spp. That Cause Root and Crown Rot in Orchard Trees. M. E. Matheron, Former Graduate Student, Agricultural Research Service, U.S. Department of Agriculture, Department of Plant Pathology, University of California, Davis 95616. S. M. Mircetich, Professor, Agricultural Research Service, U.S. Department of Agriculture, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. 69:1039-1041. Accepted for publication 11 June 1985. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1985. DOI: 10.1094/PD-69-1039.

Seedlings of Juglans ailantifolia, J. californica, J. cinerea, J. major, J. microcarpa, J. neotropica, J. nigra, J. regia, and Pterocarya stenoptera were compared with seedlings of J. hindsii and Paradox (J. hindsii × J. regia), standard rootstocks, for their relative resistance to Phytophthora citricola in artificially infested soil. Under conditions conducive for disease development, i.e., soil flooded for 48 hr biweekly, only Pterocarya stenoptera was highly resistant to Phytophthora citricola. J. ailantifolia, J. nigra, Paradox, and Pterocarya stenoptera were subsequently compared with J. hindsii for their resistance to Phytophthora citricola, P. cinnamomi, P. cryptogea, P. citrophthora, P. cactorum, and P. megasperma, J. hindsii was highly susceptible to P. cryptogea, P. citrophthora, P. cactorum, and P. megasperma only when flooded biweekly for 48 hr, but it was highly susceptible to P. citricola and P. cinnamomi in both nonflooded and periodically flooded treatments. J. ailantifolia showed significant resistance to P. cinnamomi, whereas Paradox, J. ailantifolia, and J. nigra were resistant to P. citricola and P. cryptogea under nonflooded conditions; however, this resistance was overcome by periodic flooding. Pterocarya stenoptera was highly resistant to Phytophthora citricola, P. cinnamomi, P. cryptogea, P. citrophthora, P. cactorum, and P. megasperma, even when flooded for 48 hr biweekly.