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Disease Control and Pest Management

Further Evidence for the Direct Mode of Action of Fosetyl-Al and Phosphorous Acid. M. E. Fenn, Graduate research assistant, Department of Plant Pathology, University of California, Riverside 92521; M. D. Coffey, professor, Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 75:1064-1068. Accepted for publication 3 May 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-1064.

Detached tomato leaflets were inoculated with Phytophthora capsici and floated on either water, fosetyl-Al, phosphorous acid (H₃PO₃), or metalaxyl, or one of these in combination with aminooxyacetic acid (AOA) or glyphosate. Glyphosate did not affect lesion size in any of the treatments. Compared to the water control, AOA caused a 28% increase in lesion length in leaflets inoculated with P. capsici. Fosetyl-Al (260 μg/ml), H₃PO₃ (180 μg/ml), and metalaxyl (5 μg/ml) inhibited lesion expansion, either in the presence or absence of 0.08 mM AOA. However, in the presence of AOA, lower concentrations of both fosetyl-Al (130 μg/ml) and H₃PO₃ (90 μg/ml) were less effective in reducing lesion size. With P. capsici in liquid culture, 0.08 mM AOA caused a 62% reduction in uptake of H₃PO₃ over 3 hr. A laboratory-developed mutant (H36) of P. capsici which was H₃PO₃-tolerant in vitro was also insensitive to both H₃PO₃ and fosetyl-Al in vivo.