Methods of Fosetyl-Al Application and Phosphonate Levels in Avocado Tissue Needed to Control Stem Canker Caused by Phytophthora citricola. Z. A. El-Hamalawi, Department of Plant Pathology, and C. J. Adams, Department of Statistics, University of California, Riverside 92521. J. A. Menge, Department of Plant Pathology, and C. J. Adams, Department of Statistics, University of California, Riverside 92521. Plant Dis. 79:770-778. Accepted for publication 4 April 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0770.

The efficacy of several methods of fosetyl-Al application to control avocado stem canker disease, caused by Phytophthora citricola, was evaluated under greenhouse conditions. Fosetyl-Al was applied to the canker area as paint alone (0.4 g a.i. fosetyl-Al + 1.0 ml water), combined with Tree Seal (0.4 g a.i. fosetyl-AI + 0.5 g Tree Seal + 0.5 ml water), or applied alone followed by Tree Seal on either scraped bark or bark cut in a fishscale-like pattern (30 cm along the stem). The use of fosetyl-Al as a soil drench (3.2 g a.i. fosetyl-Al per liter) was also evaluated. The most effective method was either using the fosetyl-AI:Tree Seal:water formulation on heavily scraped areas of the stem or applying fosetyl-Al alone on the bark cut in a fishscale-like pattern. Applying fosetyl-Al as a soil drench was also effective in controlling stem canker disease, but to a lesser degree than the paint application method. Phosphonate, the anionic metabolite of fosetyl-Al in plants, was quantified in the bark, leaves, and roots of treated avocado plants by high-performance liquid chromatography following the different application methods of fosetyl-Al. Application of the fosetyl-Al:Tree Seal:water formulation on heavily scraped stem areas resulted in the highest level of phosphonate residue in the canker area and was the most efficacious in controlling the stem canker pathogen. Phosphonate residue in the plant inhibited infection by P. citricola for about 6 months after its application. There was a strong negative correlation (r = -0.978) between the phosphonate level in the stem bark and the size of the stem canker caused by P. citricola.