Insecticidal Soap Reduces Infection by Two Mechanically Transmitted Plant Viruses. Thomas M. Zinnen, Department of Biological Sciences, Northern Illinois University, DeKalb, IL 60115. Judith W. Vachris, Department of Biological Sciences, Northern Illinois University, DeKalb, IL 60115. Plant Dis. 74:201-202. Accepted for publication 20 October 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0201.

Safer, an insecticidal soap containing potassium salts of fatty acids, is sprayed on plants to control insects, including aphids. We suspected that soap residue on leaves was reducing plant infection by mechanically inoculated viruses. The soap was tested for its ability to reduce infection by tobacco mosaic virus (TMV) and tobacco ringspot virus and for its ability to induce resistance to virus infection in pinto bean (Phaseolus vulgaris) and tobacco (Nicotiana tabacum ‘Xanthinc’). Amending inoculum of TMV with 0.1–10% soap reduced necrotic local lesions by 43–92%. Half-leaves dipped in 1 or 10% solutions of soap and inoculated with one of the two viruses developed 35–82% fewer lesions than opposite half-leaves dipped in water. Leaves treated on the underside with soap and inoculated on the upper side with virus and bean plants treated with soap on primary leaves and inoculated 7 days later on secondary leaves were not significantly more resistant to virus infection than controls.