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Preplant Root Treatments to Reduce the Incidence of Phytophthora Species on Dormant Apple Rootstocks. S. N. Jeffers, Department of Plant Pathology, University of Wisconsin-Madison, Madison 53706. . Plant Dis. 76:12-19. Accepted for publication 23 July 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0012.

In trials over a 6-yr period, roots of dormant MM.106 apple (Malus domestica) rootstocks were soaked in aqueous fungicide suspensions before planting to reduce the incidence of naturally occurring inoculum of Phytophthora cactorum and P. cambivora, causal agents of Phytophthora crown rot. Initially, roots were soaked for 10 min or 1 hr in 1,000 mg/L of captafol, copper hydroxide, mancozeb, or metalaxyl or in 1.05% sodium hypochlorite. In later trials, metalaxyl and copper hydroxide were compared with etridiazole, fosetyl-Al, and oxadixyl. The standard duration of soaking was 1 hr and the standard concentration of active ingredient was 1,000 mg/L, except that etridiazole was tested at 100 mg/L because the higher concentration was phytotoxic. After treatment, rootstocks were planted in sterilized or pasteurized soil and grown for 1013 wk with periodic flooding to enhance disease development and pathogen proliferation. At the end of the trials, shoot and root growth were measured, and roots of plants were assayed for Phytophthora spp. with an apple-cotyledon baiting procedure. In addition, three and five times the standard concentration of active ingredients and a 3-hr soaking duration were evaluated for potential phytotoxicity. Preplant root-soak treatments significantly reduced the number of plants from which Phytophthora spp. were detected and decreased the severity of root and crown rot. Metalaxyl was the most effective treatment, although copper hydroxide also was effective. Both treatments consistently resulted in plants that were vigorous and appeared healthy. Phytotoxicity was not a significant problem with any of the fungicide treatments.