Effects of Fosetyl-Al and Phosphorous Acid on Scoparone, a Phytoalexin Associated with Resistance of Citrus toPhytophthora citrophthora

Abstract

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 phosphorous 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 .mu.g of fosetyl-Al or 125 .mu.g 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 .mu.g of fosetyl-Al or 0-125 .mu.g of H3PO3 per milliliter, whereas lesion length in niva decreased sharply only with treatments of more than 500 .mu.g of fosetyl-Al or 200 .mu.g 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 .mu.g/ml, respectively.