Dissipation of Mefenoxam Residue in Watermelon and Soil Under Field Conditions
Mefenoxam is the systemic phenylamide fungicide, which is widely used in controlling phytophthora disease of tomato, cucumber, pepper and watermelon, etc. The dissipation behaviour of mefenoxam residues in watermelons and soils was studied. The whole watermelon, melon flesh and soil matrices of mefenoxam were analyzed by GC–NPD. At three different spiking levels mean recoveries and relative standard deviation from spiked samples in six replicated experiments for each matrix were in the range 89.6–98.2% and 1.5–8.1%, respectively. Under field conditions, mefenoxam dissipation rate was found to be faster in the whole watermelons than in the soils. The results showed that the half lives in whole watermelon and soil from Beijing were 3.9 and 10.0 days, respectively, and the half lives in whole watermelon and soil from Shanxi were 3.7 and 28.4 days.
Persistence of fungicides on cucumber (Cucumis sativus L.) in relation to downy mildew disease
Incidence and intensity of downy mildew disease in cucumber, ranged from six to seventy per cent and two to thirty five per cent respectively in different cucumber growing areas of the Punjab state. Farmers provided 6-8 sprays of the fungicides at an interval of 7-15 days to manage the downy mildew disease in cucumber. Ridomil Gold and Ridomil MZ @ 0.25 per cent were having highest persistence i.e. disease appeared only 12 days after the spray of these fungicides. Next best fungicides in the order of persistence were Acrobat and Indofil M-45 @ 0.25 per cent and downy mildew disease appeared 10 days after the spray. In case of Curzate M8 and Amistar Top @ 0.1 per cent disease appeared 8 days after the spray and Amistar @ 0.1 per cent was having least persistence i.e. disease appeared 4 days after the spray. In Equation Pro @ 0.25 per cent disease appeared on the same day and was ineffective for checking downy mildew disease. The mean recoveries of metalaxyl from samples of cucumber fruits spiked at 0.02, 0.10, 0.25, 0.50 and 1.00 mg kg -1 were found to be 76.7, 80.0, 80.7, 81.0 and 80.3 per cent and the mean deposits of metalaxyl in cucumber fruits were found to be 0.20 and 0.33 mg kg -1 after application of Ridomil MZ at 0.25 and 0.50 per cent, respectively. The metalaxyl residues were found to be below detection limit (BDL) i.e. 0.02 mg kg after 15 days of spray. Residues of metalaxyl on the day of spray itself were below the maximum residue limit (MRL) i.e. 0.50 mg kg. Therefore waiting period for metalaxyl in cucumber fruits has been suggested to be one day.
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.