VIEW ARTICLE

Resistance

Effects of Fosetyl-Al and Phosphorous Acid on Scoparone, a Phytoalexin Associated with Resistance of Citrus to Phytophthora citrophthora. U. Afek, Department of Plant Pathology and Microbiology Faculty of Agriculture, the Hebrew University of Jerusalem, P.O. Box 12, Rehovot 76100 Israel; A. Sztejnberg, Department of Plant Pathology and Microbiology Faculty of Agriculture, the Hebrew University of Jerusalem, P.O. Box 12, Rehovot 76100 Israel. Phytopathology 79:736-739. Accepted for publication 2 March 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-736.

Two Phytophthora-resistant citrus species (macrophylla and sour orange) and two Phytophthora-susceptible citrus species (rough lemon and niva) were treated with fosetyl-Al or phosphorus acid and compared for production of scoparone and symptoms of infection of Phytophthora citrophthora. In macrophylla, sour orange, and rough lemon, concentrations of scoparone were two- to fourfold greater in inoculated branches treated with 300 µg of fosetyl-Al or 125 µg of phosphorous acid (H₃PO₃) per milliliter than in inoculated, untreated branches. Lesion length in these three species decreased more rapidly than in niva after treatments with 0–300 µg of fosetyl-Al or 0–125 µg of H₃PO₃ per milliliter, whereas lesion length in niva decreased sharply only with treatments of more than 500 µg of fosetyl-Al or 200 µg of H₃PO₃ per milliliter. Treatment with fosetyl-Al and H₃PO₃ did not influence scoparone concentrations in niva bark. ED₅₀ values of fosetyl-Al and H₃PO₃ for mycelial growth of P. citrophthora were 55 and 7 µg/ml, respectively.