Metalaxyl: persistence, degradation, metabolism, and analytical methods.

Metalaxyl is a systemic fungicide used to control plant diseases caused by Oomycete fungi. Its formulations include granules, wettable powders, dusts, and emulsifiable concentrates. Application may be by foliar or soil incorporation, surface spraying (broadcast or band), drenching, and seed treatment. Metalaxyl registered products either contain metalaxyl as the sole active ingredient or are combined with other active ingredients (e.g., captan, mancozeb, copper compounds, carboxin). Due to its broad-spectrum activity, metalaxyl is used world-wide on a variety of fruit and vegetable crops. Its effectiveness results from inhibition of uridine incorporation into RNA and specific inhibition of RNA polymerase-1. Metalaxyl has both curative and systemic properties. Its mammalian toxicity is classified as EPA toxicity class III and it is also relatively non-toxic to most nontarget arthropod and vertebrate species. Adequate analytical methods of TLC, GLC, HPLC, MS, and other techniques are available for identification and determination of metalaxyl residues and its metabolites. Available laboratory and field studies indicate that metalaxyl is stable to hydrolysis under normal environmental pH values, It is also photolytically stable in water and soil when exposed to natural sunlight. Its tolerance to a wide range of pH, light, and temperature leads to its continued use in agriculture. Metalaxyl is photodecomposed in UV light, and photoproducts are formed by rearrangement of the N-acyl group to the aromatic ring, demethoxylation, N-deacylation, and elimination of the methoxycarbonyl group from the molecule. Photosensitizers such as humic acid, TiO2, H2O2, acetone, and riboflavin accelerate its photodecomposition. Information is provided on the fate of metalaxyl in plant, soil, water, and animals. Major metabolic routes include hydrolysis of the methyl ester and methyl ether oxidation of the ring-methyl groups. The latter are precursors of conjugates in plants and animals. In soils the most relevant metabolite is the metalaxyl acid, which is formed predominantly by soil microorganisms. Plant uptake, microbial degradation, photodecomposition, and leaching are the major route of metalaxyl dissipation. It has a tendency to migrate to deeper soil horizons with a potential to contaminate groundwater, particularly in soils with low organic matter and clay content. Therefore, precautions should be taken for the continuous application of metalaxyl to crops. If use of metalaxyl is greately increased, the risk of occurrence in groundwater must be reassessed, as by monitoring studies in the most vulnerable areas in main use regions. The R-isomer of metalaxyl (mefenoxam) has recently been registered as the only active compound. Therefore, quantitative studies on the fate of this specific isomer are needed, including appropriate analytical methods. As the use rates of mefenoxam are approximately one-half those recommended for metalaxyl and mefenoxam dissipates more rapidly, concerns for mefenoxam reaching groundwater are even less justified.