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Pathogenicity and Relative Virulence of Nine Phytophthora spp. from Kiwifruit. K. E. Conn, Former graduate student, Department of Plant Pathology, University of California, Davis 95616; W. D. Gubler(2), S. M. Mircetich(3), and J. K. Hasey(4). (2)(3)Extension plant pathologist, and USDA, ARS plant pathologist, respectively, Department of Plant Pathology, University of California, Davis 95616; (4)Farm advisor, University of California, 142-A Garden Highway, Yuba City 95992. Phytopathology 81:974-979. Accepted for publication 10 April 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-974.

Nine species of Phytophthora, including P. cactorum, P. citrophthora, P. cryptogea, P. drechsleri, P. megasperma, and four unidentified Phytophthora spp., were isolated from kiwifruit vines (Actinidia deliciosa) affected with root and crown rot in California commercial orchards. P. citrophthora, P. cryptogea, and P. megasperma were the most commonly recovered Phytophthora spp., whereas the remaining species were recovered sporadically. Kiwifruit vines infected by Phytophthora spp. exhibited poor terminal growth, stunted, chlorotic, and drooping leaves, and various degrees of dieback, including complete vine death. The highest incidence of diseased kiwifruit vines usually occurred on sites subjected to prolonged and repeated soil saturation due to poor drainage, flooding, or over-irrigation. In greenhouse experiments with artificially infested U.C. mix, seedlings of kiwifruit cultivars Hayward, Bruno, and Abbott developed various degrees of root rot and/or crown rot within 3 mo when periodically flooded for 48-h intervals every 2 wk. P. cryptogea and two of the unidentified Phytophthora spp. (3 and 4) caused severe root rot (90100% of root mass rotted) and crown rot (58100% of seedlings affected), regardless of the kiwifruit cultivar. Isolates of P. citrophthora caused moderate to severe root rot (5093%) on all cultivars, but varied greatly in their ability to cause crown rot (0100%). P. megasperma caused moderate to severe disease on seedlings of the cultivars Bruno and Abbott, inducing 6188% and 7989% root rot, and 850% and 75100% crown rot, respectively. However, P. megasperma was less virulent to seedlings of the cultivar Hayward, causing 2630% root rot and no crown rot. P. drechsleri and unidentified Phytophthora sp. 1 were weakly to moderately virulent on all three kiwifruit cultivars, causing 2657% and 2848% root rot, and 017% and 08% crown rot, respectively. P. cactorum and unidentified Phytophthora sp. 2 were avirulent and caused no measurable disease in any of the three kiwifruit cultivars.