Effect of Postinfection Application of Phosphorous (Phosphonic) Acid on the Incidence and Sporulation of Plasmopara viticola on Grapevine. T. J. Wicks, South Australian Department of Agriculture, GPO, Adelaide, South Australia, 5001. P. A. Magarey, M. F. Wachtel, and A. B. Frensham. South Australian Department of Agriculture, GPO, Adelaide, South Australia, 5001. Plant Dis. 75:40-43. Accepted for publication 18 June 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0040.

Phosphorous acid (H₃PO₃) applied at 1.2 g/L up to 12 days after infection reduced the incidence and severity of Plasmopara viticola. When applied up to 13 days after infection, H₃PO₃ reduced sporulation. In some experiments, the postinfection activity of H₃PO₃ was better than metalaxyl, while in other experiments it was less effective. Both fungicides applied after infection reduced the incidence and severity of disease on leaves and flower clusters, indicating that postinfection applications should reduce the production of inoculum and the consequent spread of disease within a vineyard. In most cases, the addition of copper oxychloride did not affect the efficacy of H₃PO₃.