Bioassay and other laboratory methods for testing formulations of potato blight fungicides

Abstract

SUMMARYThree laboratory methods are described for comparing formulations of potato blight fungicides; all use the upper surfaces of leaflets detached from glasshouse‐grown King Edward plants, and artificial rain in some form.Rain‐fastness (tenacity) was quickly estimated by measuring advancing acute contact angles of distilled water (representing rain) on dry spray deposits. Tenacity itself was measured by 'rain‐washing' deposits with 0·65 in. of tap water in 30 min. (light 'rain') or up to 2·5 in. in 40 mm. (heavy 'rain'). Contact angles and tenacities of copper oxychloride deposits were usually positively correlated. In bioassay tests, deposits were washed with 2·5 in. of heavy 'rain', inoculated with sporangia of Phytophthora infestans (Mont.) de Bary, and later classed as infected or not infected; results were analysed by the probit method. With copper oxychloride and triphenyl tin (fentin) acetate, differences in effectiveness of 2·5 times or more were usually significant when fifty to sixty leaflets were used per formulation.