Phosphonate Fungicides for Control of Diseases Caused by

In greenhouse experiments, seedlings of Persea indica, an avocado relative, planted into soil naturally infested with Phytophthora cinnamomi did not develop root rot when treated with a foliar spray or soil drench of potassium phosphonate or fosetyl-aluminum (trade name = Aliette®). Potassium phosphonate was prepared by adding potassium hydroxide to phosphorous acid in water. Potassium phosphonate and fosetyl-aluminum also controlled stem canker of bare-rooted seedlings of P. indica caused by P. citricola. Phosphonate incorporated into culture media was more inhibitory toward growth of Phytophthora species than was fosetyl- aluminum. Two days after treating plant tissue with fosetyl-aluminum, more than 95 % of the parent compound was degraded to phosphonate. Concentrations of phosphonate in plants treated with fosetyl-aluminum or with potassium phosphonate were sufficiently high to directly inhibit growth of Phytophthora. In the laboratory, use of chemical mutagenesis procedures led to the development of pathogenic isolates of Phytophthora which were resistant to high concentrations of phosphonate. In the field, a breakdown in control by fosetyl-aluminum of root rot, caused by P. cinnamomi, has been recorded in France. The risk of this occurring with avocados under California conditions needs to be assessed if these valuable fungicides are to be used intensively and over extended periods of time