By agronomist Isabel Pérez, Member of the Technical Commission of Pesticides College of Agricultural Engineers of Chile.
Copper is not only an essential trace element for the survival of organisms, but has also been used for decades in agriculture, to control different types of phytopathological problems caused by oomycetes, fungi and bacteria. Due to this biocidal characteristic, this element constitutes an important alternative to consider for integrated management, and in this case, in the case of organic agriculture, this alternative turns out to be an essential control strategy.
This last aspect, however, may have negative consequences in the medium term, because copper, being a heavy metal, before productive regimes that base their control strategy on this type of element, cause their constant accumulation in surface profile of the soil, constituting an incongruous situation with respect to the principles pursued by organic farming.
Historically, copper-based antimicrobial compounds (CBCs) have been used in agriculture due to their relatively high toxicity to target plant pathogens, their relatively low cost, low toxicity to mammals in the case of less leachable compounds, as well as their chemical stability. However, there have been not few findings from different research worldwide that account for a number of undesirable effects associated with the use and in many cases «abuse» of CABC.
Among these we can mention:
- development of copper resistant bacterial strains,
- accumulation of copper in the soil,
- negative effects on soil biota (decrease and alteration of macro and microbiota composition),
- decreased bioavailability of nutrients,
- alteration of quality parameters in food, among others.
Given this abundant scientific evidence, different entities associated with global agriculture are exerting pressure (through different instances), so that those responsible for generating local regulatory regulations define and limit the use of these compounds. What is to be restricted? , the types of CABC according to their degree of safety in relation to the consumer and the environment, as well as the total volumes of such compounds used per hectare/year.
To date, in the case of Chile, there are a total of 81 CABC (cuprous oxide, cupric oxide, copper hydroxide, tribasic copper sulphate, Bordelase broth, pentahydrate copper sulphate, copper dibasic sulphate, basic copper sulphate, copper dibasic carbonate and copper 8-quinolate, the latter two, registered exclusively for use in wood treatment in the forest), all duly registered with the Agricultural and Livestock Service, however, none of them has in the current regulations Maximum Residue Limit Values (LMRs) explicitly established (Resolution 892 EXEMPTED, approves Technical Standard 209, which establishes the Maximum Residue Limits of Pesticides in Food, currently under review).
In accordance with this situation and as specified by our regulations, the establishment of the omitted MRL values should be done by consulting the databases of Codex Alimentarius, United States of America (US EPA: United State Environmental Protection Agency) and/or European Union (EFSA: European Food Safety Authority, European Union). Unfortunately to date, Codex does not specify LMRs values for CABC, while, as in the case of EPA and EFSA, none of them matches the CBCs authorized for use in each of the territories that each of them regulates or regulates.
In the case of the European Union, decision-making with regard to more strictly regulating the use of CBCs, started in 2009 (Commission Directive 2009/37/EC of 23 April 2009), by limiting the types of copper compounds approved. This regulation restricted only 5 CABC options, including copper hydroxide, copper oxychloride, Bordeaux broth or Bordeaux mixture, tribasic copper sulphate and copper oxide, options from which a producer must choose, according to their requirements and the ability of such compounds to release copper ions in solution. Copper sulphate form is excluded due to its extreme solubility and consequent toxicity to the applicators and the environment.
Subsequently, this regulation has established new regulatory parameters, in this case, with respect to the volumes of copper authorized to be used in one hectare treated/year. Initially several countries authorised an applied volume of 6 kg per hectare for the maximum period of one year, however, this standard has become much more stringent (Commission Implementing Regulation (EU) 2018/1981 of 13 December 2018) and has established, to date, a maximum authorised volume of 28 kg of copper metal per hectare, for a period of not less than 7 years (or the equivalent of 4 kg of copper/ha/year).
However, given the pressure from the EU’s agricultural and scientific environment, the standard has included a new additional regulation including, among other points, the recommendation that in those localities where there is some other contribution of exogenous copper to the work of the productive agro-ecosystem, such additional input quantity of copper is deducted from that authorised by producers. This regulation, if implemented in Chile, specifically in the case of the Region of O Higgins, how would it affect… In our regulatory context, further analysis that considers particularities is required and necessary.
Although in the case of the EU the CBCs mentioned above are authorised for manufacture, marketing, use and transport within the EU territory, in accordance with the prevailing regulations, these compounds have the categorisation of «candidates for replacement»due to its persistent and toxic condition, the expiry date of which was set at 31 December 2025 (Commission Implementing Regulation (EU) 2020/1093 of 24 July 2020).
In the case of EPA (USA), the regulation is considerably more permissive than the one described above for EFSA, both with respect to the types of CABC explicitly authorised, their LMRs values and also with respect to the volumes authorised to apply/hectare year (it does not set limits). This regulation can be attractive to adopt as a model to follow, mainly from the commercial point of view, by allowing a greater number of CABC. However, many of the compounds authorized here have robust scientific evidence recommending their use ban due to the negative impact of their use on the agro-ecosystem.
As can be concluded so far, the use of copper in agriculture, without neglecting its great utility in agriculture as a biocide and fertilizer, is quite controversial, especially in the case of organic farming, where unfortunately in a relatively short time, could become banned as is currently the case in countries such as Denmark, Finland, the Netherlands, Norway and Switzerland (these countries have also banned the use of copper in integrated pest management programmes in conventional production systems).
However, it is not only necessary to establish clear local regulations defining the most safe CABC options for use in agriculture (regardless of the type of production system to be treated) and also limiting the maximum contribution of kg of metallic copper per hectare/year, but also that with these regulatory tools in force, strategies are elaborated, in a multidisciplinary manner, for the efficient use of the CABC and thus prolong the useful life of these compounds, regardless of the area in the country in which they are required (localities with additional copper input from pollution and/or contaminated irrigation water).
Strategies for preventing, mitigating or limiting the negative effects of CBCs should include:
- its optimised use of copper in phenological states susceptible to infection;
- use of resistant/tolerant varieties or cultivars;
- establishment of crops in benign agro-climatic zones;
- timely cultural practices (making the agro-ecosystem inhospitable for pathogens);
- use of disease prognostic models if available;
- use of CABC with higher fixing capacity;
- calibration of applicator equipment (alterations in applied doses promote the development of copper resistant strains);
implementation of soil bioremediation;
- development and evaluation of alternative compounds to copper (e.g. biocontrolators, an option in which Chile has an enormous research and development advantage that is already being implemented).
As it is possible for the reader to conclude, the use of copper as well as the problems associated with the uninformed use of the CABC and the regulations that regulate it, are a permanent, silent and very little recognized time bomb that, in the event of failure to take the necessary measures in the short term, it will cause the loss of this valuable tool for managing diseases affecting crops of great economic importance as well as eventually, the collapse of organic or biodynamic production systems.