Effects of the fungicide mixture mancozeb, metalaxyl-M, and chlorothalonil on the degradation of $^{14}C-MCPA$ in soil

Nguyen, Kim Thu; Schäffer, Andreas (Thesis advisor); van Dongen, Joost Thomas (Thesis advisor); Smith, Kilian (Thesis advisor)

Aachen : RWTH Aachen University (2023)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2023


Objective: In agriculture, the use of pesticides including herbicides, fungicides, and insecticides has become indispensable. Taking into consideration that up to now, only the single active ingredients and commercialized pesticide formulations are examined for their environmental fate and effects as part of the approval procedure for plant protection products, this study investigated the impact of the fungicide mixture mancozeb, metalaxyl-M, and chlorothalonil on the degradation of 4-chloro-2-methylphenoxyacetic acid (MCPA) degradation. This is relevant, since it is the rule rather than the exception that multiple pesticides are applied, either in tank mixtures prepared by the farmers or as an application series during the agricultural season. Method: The degradation of MCPA in the presence and absence of the fungicides mancozeb, metalaxyl-M, and chlorothalonil at recommended field rates in soil was studied in stationary test systems under controlled laboratory conditions. At appropriate time intervals, the soil was extracted and analyzed for MCPA and its transformation product 4-chloro-2-methylphenol (4C2MP) using chromatographic techniques. The levels and types of soil bound non-extractable residues, volatile organic compounds as well as 14CO2 was included in the mass balance. The dissipation kinetics of MCPA were modelled, and the dissipation times (DT50) determined. Results: The half-life values of 14C-MCPA parent compound derived from 14C-ER in soil increased from 1.61 - 1.88 days for the controls to 2.51 - 3.49 days when fungicides were co-applied. This decrease in the dissipation rates was further supported by chromatographic analyses, showing that 4C2MP was formed in considerably lower amounts (≤ 1 % AR) when the fungicides were co-applied. Overall, the fate and behaviour of MCPA was altered by the fungicide application. Conclusion: The data provide evidence that a fungicide mixture consisting of mancozeb, metalaxyl-M, and chlorothalonil, each applied according to their recommended field rates, either as a tank mix, spray series or as combination of both, reduced the degradation of MCPA. This work is consistent with those few other studies that report pesticide mixtures as affecting the degradation of other pesticides present in the soil. Importantly, this study shows that this is the case even at realistic fungicide application levels. Therefore, it is proposed that the authorization of plant protection products should include the testing of the pesticides as part of those mixtures that are applied during typical agricultural practice.


  • Department of Biology [160000]
  • Chair of Environmental Biology and Chemodynamics [162710]