Further Evidence for the Direct Mode of Action of Fosetyl-Al and Phosphorous Acid

Abstract

Deteached 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 .mu.g/ml), H3PO3 (180 .mu.g/ml), and metalaxy (5 .mu.g/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 .mu.g/ml) and H3PO3 (90 .mu.g/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.