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Efficacy of Two New Systemic Fungicides and Ethazole for Control of Phytophthora Root Rot of Rhododendron, and Spread of Phytophthora cinnamomi in Propagation Benches. L. Englander, Assistant professor, Department of Plant Pathology-Entomology, University of Rhode Island, Kingston, 02881; J. A. Merlino(2), and J. J. McGuire(3). (2)Graduate research assistant, Department of Plant Pathology-Entomology, University of Rhode Island, Kingston, 02881; (3)Professor, Department of Plant and Soil Science, University of Rhode Island, Kingston, 02881. Phytopathology 70:1175-1179. Accepted for publication 30 May 1980. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-1175.

Three fungicides, SN 66752, CGA 48988, and ethazole were tested for efficacy against Phytophthora root rot of rhododendron, which frequently causes high levels of mortality on container-grown plants. Monthly drenches of CGA 48988 (75 μg/ml), initially applied prior to inoculation of container-grown rhododendrons, moderately controlled root rot, and were more efficacious than drenches with ethazole (300 μg/ml) or SN 66752 (1000 μg/ml). However, level of control decreased with increasing susceptibility of the cultivars. When applied after inoculation, ethazole exhibited very limited therapeutic activity, and SN 66752 and CGA 48988 were ineffective. These fungicides would have no practical value when used on container plants that had been previously infected. Phytophthora cinnamomi spreads among rooted cuttings in propagation benches, but many of the plants infected in this way may remain symptomless until late in the following season. Phytotoxicity of fungicides was studied to determine the practicality of applying such materials to young plants during propagation. Monthly applications of CGA 48988 or SN 66752 did not inhibit rooting or subsequent root development, whereas ethazole was phytotoxic.