Behavior of metalaxyl and mancozeb residues in soils and tomato

Abstract

The present investigation on “Behaviour of metalaxyl and mancozeb residues in soil and tomato” was taken up with the main objectives of finding the extent of adsorption and degradation of fungicide residues in soils, their uptake and persistence in tomato plants. The studies were conducted on a systemic fungicide metalaxyl and a contact fungicide mancozeb along with its major metabolite ETU interacting with tomato plant and three main types of soil from Bangalore, Chettalli and Hiriyur representing the different textural classes. The salient features of the investigation are summarized as below. Adsorption of metalaxyl was mainly influenced by the clay content of the soils. Metalaxyl adsorption in soils decreased in the order of Hiriyur> Chettalli> Bangalore, which is also the order of decreasing clay content. Freundlich equation provided a good fit (R2>0.9) for the adsorption data metalaxyl and ETU in all the soils. The sorption coefficients for organic carbon (Koc) were higher for metalaxyl than ETU in all the soils and ranged from 362 to 2275 μg ml-1 for metalaxyl and 128 to 581 μg ml-1 for ETU. The degradation pattern of metalaxyl, mancozeb and ETU residues indicated a close correspondence to first order exponential degradation kinetics in soils and mainly influenced by soil moisture and temperatures. Slower degradation of metalaxyl was noticed in the soils and their half – life values were higher than mancozeb and ETU as evident by wide range of half – life values from 41.24 to 165.11 days. In case of metalaxyl Hiriyur soil was found to be superior in degrading the metalaxyl. Lower persistence of mancozeb and ETU was observed in the soils resulting in rapid rate of degradation at smaller half – life values as compared to metalaxyl indicating the faster degradation of mancozeb and ETU. In mancozeb treated soils, the ETU formation was increased up to 30 days of incubation and thereafter it declined. The plant studies have indicated that maximum concentration of metalaxyl and ETU residues in tomato plants following uptake from soil was attained after 15 and 20 days of soil application respectively. The residues were however residue further and reached below the prescribed MRL in fruits by harvest itself. The degradation of soil retained fungicide residues followed the first- order exponential reaction (R2>0.9) and metalaxyl persistence was higher than mancozeb. The half-life values for metalaxyl were in the range of 6.52 to 9.57 days, while mancozeb ranged from 9.74 to 12.41 days for different treatments. The rapid disappearance of metalaxyl and mancozeb residues to the extent of 59.2 to 72.4 and 3.81 to 48.3% was found within 10 days of fungicide applications at the two application rates. Foliage accumulated comparatively higher initial residues of fungicides than fruits following foliar applications. Metalaxyl residues dissipated at higher rate of residue decay in foliage as compared to fruits. A reverse order on dissipation of mancozeb residues was found in foliage and fruits. The half – life of metalaxyl in foliage from 3.13 to 4.14 days and 7.82 to 8.75 days for tomato fruits as compared to the mancozeb half-life of 13.87 and 13.56 days for foliage and 5.23 and 6.95 days for fruits from 0,2 and 0.4% metalaxyl-MZ spray respectively. The persistence of ETU in foliage continued beyond 40 days but fruits were devoid of any detectable residues. The degradation rate constants were accordingly higher in field conditions (75.63 x 10-3 to 127.85 x 10-3 day-1) compared to laboratory incubation studies (10.24 x 10- 3 to 46.99 x 10-3 day-1). Fungicide residues in general were decreased upon washing and cooking of tomato fruits after 5 and 10 days of spray application. The rate of loss of residues were almost same from the two application rates upon washing followed by cooking while the intensity of dislodging the residues being severe from washing followed by cooking. Further the dislodging effects were more pronounced on the less aged residues. Decontamination processes of washing followed by cooking removed as high as 78.3 to 78.86% metalaxyl; 74.6 to 78.4% mancozeb and 44.5 to 48.3% ETU residues.