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Relative Resistance of Thirteen Apple Rootstocks to Three Species of Phytophthora. G. T. Browne, Plant pathologist, USDA-ARS, Department of Plant Pathology, University of California, Davis 95616; S. M. Mircetich, Research plant pathologist, USDA-ARS, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 83:744-749. Accepted for publication 8 January 1993. 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, 1993. DOI: 10.1094/Phyto-83-744.

Thirteen apple rootstocks (Antonovka [Ant.] 313, Budagovsky [Bud.] 490, Bud.118, Bud.9, M.4, MM.111 EMLA, MM.106 EMLA, M.7 EMLA, M.26 EMLA, M.9 EMLA, Mark, Polish [P.] 18, and a domestic seedling [Malus domestica]) were evaluated for resistance to root and crown rots caused by Phytophthora cactorum, P. cambivora, and P. cryptogea in artificially infested soil and for resistance to canker development caused by P. cactorum and P. cambivora in excised and intact stems. In soil infested with P. cactorum, M.9 EMLA, Mark, Bud.118, and Bud.9 were highly resistant (mean crown circumference girdled 211%); MM.106 EMLA, Ant.313, and the domestic seedling were highly susceptible (mean crown circumference girdled 7496%); and the remaining rootstocks were intermediate. With P. cambivora, Mark and Bud.118 were highly resistant (mean root rot 9%); Bud.9, M.7 EMLA, and P.18 were intermediate, and the other rootstocks were moderately to highly susceptible (mean root rot 4798%). With P. cryptogea, most rootstocks were relatively resistant (mean root rot 110%), except M.4, MM.111 EMLA, Ant.313, and P.18 (mean root rot 1842%). After stem inoculations with P. cactorum, mean canker lengths in excised and intact rootstock stems were correlated positively with mean crown rot lengths in corresponding apple rootstocks grown in infested soil (r = 0.590.79, P = 0.050.004). With P. cambivora, however, mean canker lengths from stem assays of resistance did not correlate significantly with mean crown rot lengths from infested soil assays of resistance (P = 0.930.11). Relative resistance to Phytophthora spp. in apple rootstocks can vary by Phytophthora sp. and by method of evaluation.