Efficacy and In Vitro Activity of Selected Fungicides for Control of Phytophthora cactorum Collar Rot of Apple. E. A. Brown II, Graduate Student, Department of Plant Pathology and Plant Genetics, University of Georgia, Athens 30602. F. F. Hendrix, Professor, Department of Plant Pathology and Plant Genetics, University of Georgia, Athens 30602. Plant Dis. 64:310-312. Accepted for publication 10 October 1979. Copyright 1980 American Phytopathological Society. DOI: 10.1094/PD-64-310.

Phytophthora cactorum was consistently recovered from the root systems of nursery transplants and was the primary inoculum source in collar rot areas in Georgia. Captafol as a root dip was more effective in reducing the recovery of P. cactorum from MM106 and M26 apple stocks than ethazole, benomyl, maneb, PCNB, and copper sulfate. Captan, PCNB, ethazole, benomyl, and captafol at 100 ppm significantly reduced the growth of P. cactorum in vitro. Fungicide trunk sprays and drenches did not significantly reduce the incidence of collar rot in the field over a 3-yr period. Copper sulfate was inadequate for control because of phytotoxicity in the root dip treatments and burr knot formation on trees drenched in the field. Incidence of collar rot was more severe on trees under stress.