VIEW ARTICLE

Disease Control and Pest Management

Quantification of Phosphonate and Ethyl Phosphonate in Tobacco and Tomato Tissues and Significance for the Mode of Action of Two Phosphonate Fungicides. M. E. Fenn, Department of Plant Pathology, University of California, Riverside 92521-0122, Current address: Forest Fire Laboratory, U.S. Department of Agriculture Forest Service, Pacific Southwest Forest and Range Experiment Station, Riverside, CA 92507; M. D. Coffey, Department of Plant Pathology, University of California, Riverside 92521-0122. Phytopathology 79:76-82. Accepted for publication 20 April 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-76.

Quantification of ethyl phosphonate and phosphonate (HPO₃², phosphite) in plant tissues treated with the phosphonate fungicides fosetyl-Al and potassium phosphonate was achieved using high performance ion chromatography. Phosphonate also was quantified by scintillation counting using tritium-labeled HPO₃². Lesion length in HPO₃²-treated tomato leaflets inoculated with Phytophthora capsici and containing 8.9 mM phosphate and 78-110 g/g fresh weight (fr. wt.) of HPO₃² was reduced by 61%. Tomato leaflets treated with 160 μg/ml of fosetyl-Al for 48 hr contained 88 μg/g fr. wt. of HPO₃^2? but only 3 μg/g fr. wt. of ethyl phosphonate. Because the in vitro EC₅₀ value for P. capsici with media containing 5 mM potassium phosphate was 77 g/ml of H₃PO₃², these combined results support a direct mode of action for HPO₃². Likewise with P. parasitica var. nicotianae on tobacco, the HPO₃² content of seedlings treated with 390 μg/ml of HPO₃² or 1,000 μg/ml of fosetyl-Al (279 μg/g and 308 μg/g, respectively) was sufficient to account for disease control through a direct mode of action. Using chemical mutagenesis, strains of P. capsici and P. parasitica var. nicotianae were obtained which grew on 0.5% cornmeal agar containing 878 μg/ml of HPO₃². One of the mutant strains of P. parasitica var. nicotianae killed tobacco seedlings containing 484 μg/g fr. wt. of HPO₃², whereas plants inoculated with the parental wild-type isolate were symptomless in the presence of 215 μg/g fr. wt. of HPO₃². Uptake of HPO₃² by P. parasitica var. nicotianae was inhibited 77-80% over 4 hr when α-aminooxyacetic acid (AOA) was added to culture media. In the presence of AOA in vivo, 390 μg/ml of HPO₃² protected tobacco plants from infection with P. parasitica var. nicotianae, whereas 195 μg/ml was ineffective. These data add further support to the concept that both potassium phosphonate and fosetyl-Al, through the activity of HPO₃², have a direct mode of action against Phytophthora species in their hosts.