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Phytotoxicity of the Fungicide Metalaxyl and Its Optical Isomers

Metalaxyl is a systemic fungicide commonly used in citrus production to control Phytophthora root rot and foot rot. When applied as a drench, injury was observed on newly planted, young citrus trees. The visual injury symptoms ranged from light- to bright-yellow leaves. It was learned that the commercial formulation of metalaxyl contained various isomers and that these isomers may vary in phytotoxic effects on citrus leaves. The objective of this study was to determine the difference in herbicidal activity between the two optical isomers of metalaxyl on pepper plants and citrus leaves. The phytotoxicity of the fungicide metalaxyl and its optical isomers (CGA76538, S⁺; CGA76539, R^-) was determined using a pepper seedling growth bioassay and by measuring protein synthesis as estimated by the incorporation of ¹⁴C-leucine by citrus mesophyll cells. The two isomers and metalaxyl differed in their herbicidal activity to pepper plants and citrus cells. Pepper seedlings treated with R^- had significantly higher mean fresh weight than plants treated with metalaxyl or S⁺ at 0.1, 1.0, 10.0, and 100 ppm. Protein synthesis, as measured by the inhibition of ¹⁴C-leucine incorporation by citrus mesophyll cells, also was inhibited more by metalaxyl and the S⁺ isomer than by the R^- isomer. After 30 min of incubation at 100 μM, the R^- isomer inhibited ¹⁴C-leucine incorporation by 29%, whereas incorporation of ¹⁴C-leucine in the metalaxyl and the S⁺ isomer treatments was higher (46 and 81%, respectively). Similarly, the highest ¹⁴C-leucine uptake at 60 min was obtained by R^- at all concentrations. The assays showed that the R^- and S⁺ isomers differ in their biological activity as expressed by weight loss of pepper plants and inhibition of protein synthesis and that the S⁺ isomer is responsible for the phytotoxicity of metalaxyl. The findings in this study show that the phytotoxicity of metalaxyl was due to the presence of the S⁺ optical isomer. Removal of this isomer from metalaxyl has enabled the continued use of this fungicide for control of foot rot and root rot in citrus.