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Phosphonate Levels in Avocado (Persea americana) Seedlings and Soil Following Treatment with Fosetyl-Al or Potassium Phosphonate. D. G. Ouimette, Graduate Research Assistant, Department of Plant Pathology, University of California, Riverside 92521-0122. M. D. Coffey, Professor, Department of Plant Pathology, University of California, Riverside 92521-0122. Plant Dis. 73:212-215. Accepted for publication 5 October 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-0212.

The levels of ethyl phosphonate and phosphonate in avocado seedlings and soil were determined using high-performance ion chromatography at 1, 2, 4, 6, and 8 wk following foliar or soil applications of either 3 mg/ml of fosetyl-Al or 2.1 mg/ml of potassium phosphonate. After soil treatment with either potassium phosphonate or fosetyl-Al, phosphonate persisted in soil for 2 and 4 wk, respectively. With fosetyl-Al, low levels of ethyl phosphonate were present in soil, roots, and stems 1 wk after application, but none was detected thereafter. In contrast, no ethyl phosphonate residues were detected in either soil or avocado tissue 1 wk following foliar application of fosetyl-Al. Soil treatment with both potassium phosphonate and fosetyl-Al resulted in much higher phosphonate levels being present in all tissues compared with foliar treatment (up to 78 and 94 times more in the root samples following potassium phosphonate and fosetyl-Al treatment, respectively). Following both soil and foliar applications of the two fungicides, high phosphonate levels were maintained in avocado tissues for the 8-wk period of the experiments, suggesting that phosphonate is stable in plants. The phosphonate levels found in roots after either soil or foliar applications were sufficiently high to account for a direct antifungal effect in controlling avocado root rot caused by Phytophthora cinnamomi.