Copper and sulfur-based compounds

Why is copper not banned in organic farming?

The relentless praise of Bordeaux mixture made of lime and copper sulphate in organic farming is simply due to the role of this broad-spectrum pesticide in combating fungal diseases (mildew, rust, peach blight, etc.) and bacterial diseases, the only really effective pesticide in the very limited list of products approved for this sector. The use of copper sulphate in organic farming takes advantage of the versatility of high doses and repeated applications without too much consideration for the auxiliary fauna. This is particularly the case in arboriculture to combat diseases such as scab or in viticulture against excoriosis and black rot.

Organic apple growing uses high doses of copper sulphate, although this is never mentioned on the display. A nice coloured organic apple without scab has mostly benefited from copper treatments. The use of high doses of copper sulphate compounds is rarer in integrated farming, with copper sulphate being used in alternation with other synthetic fungicides to reduce treatment doses and resistance. Without copper sulphate, the whole ideological edifice of organic agriculture would collapse, with arboriculture and viticulture in the lead. This is why the organic lobby is working hard to ensure that the use of copper sulphate-based products is not banned (1).

Following the action of organic farming lobbyists, the use of copper pesticides has been extended for 7 years from 1 January 2019 in the European Union. The European Food Safety Agency (EFSA) had however underlined the risks presented by the use of copper-containing compounds in agriculture, particularly in terms of soil pollution. In order to reduce this risk of pollution, a compromise was found by means of an adjustable reduction in the dose per hectare year by year, which is proving very difficult to implement as no one knows in advance what the weather conditions will be like next year.

For organic vineyards, the limit on copper use is 6 kilos per hectare per year smoothed over five years. This allows operators to adjust the dose above 6 kg in bad years provided that the dose is reduced in other years. In organic farming, there is currently no other substitute for copper to combat downy mildew, which can lead to considerable crop losses if left uncontrolled.

Before discussing the environmental and human health risks of excessive copper treatments, it is important to emphasise that low-dose copper is an essential element for plant life, particularly for nitrogen functionality (2) and carbohydrate metabolism. However, the doses used in organic farming are much higher than the plants’ needs. For example, the copper needs of a vine are estimated at between 30 and 100 gr/ha/year. For vines, a single treatment with Bordeaux mixture results in 300 to 5000 g/ha of copper metal. It is well known that it is the excess of copper that is toxic.

Copper is not toxic to humans as long as its concentration in food remains limited. It does not accumulate in the body except in the case of genetic abnormality or dietary overdose with a preponderant accumulation in the liver. It is an essential trace element involved in numerous enzymatic reactions, the functioning of the immune system, the formation of red blood cells, the production of melanin and gene transcription. It is also an antioxidant. A balanced diet should therefore contain copper, but in very small quantities. The recommended daily allowance is between 1.5 and 2 mg. Traces of copper in food from agricultural processing are therefore acceptable as long as the total intake of this heavy metal (including natural and industrial sources) does not exceed a certain value estimated at 5 mg/day by the European Commission. However, there are few studies verifying copper doses on vegetables, fruits and industrial preparations from organic farming.

On the other hand, sheep can easily become victims of copper poisoning. The storage of copper in the liver of sheep is related to the low capacity of the biliary secretion to eliminate copper. Liver levels in excess of 500 ppm dry weight are generally considered toxic. For this reason, it is advisable not to graze sheep in grassed areas that have been treated with copper, such as former orchards and vineyards.

a) The phytosanitary products containing copper and/or sulphur registered for use in organic farming are : Copper hydroxide (Cu(OH)₂), cuprous oxide (Cu₂O), copper sulphate (CuSO₄), micronised sulphur and sublimed sulphur.

The use of copper in organic farming; advantages and disadvantages

Sulphur and copper are among the first fungicides used in agriculture that have the particularity of being multisite (synthetic fungicides such as mancozeb and chlorotalonil belong to this category). This means that they attack the parasitic fungus by acting on several sensitive cell sites. They are moderately effective and are very leachable (for Bordeaux mixture, it is often necessary to use several kg per hectare). Modern fungicides, which are the result of biological research (often copying natural molecules), only act on one part of the fungi’s metabolism and are very effective with small quantities per ha (e.g. 200 g) with a longer residual effect (3 to 4 weeks) than the old synthetic molecules. However, due to their unisite mode of action (a), resistance can occur rapidly if precautions are not taken in their use (alternation of products, mixtures of old and recent molecules…).

In the wet season, copper is ineffective against downy mildew, which is already present on crops and develops rapidly on potatoes, tomatoes and vines, and against the cryptogamic infections that swarm on fruit trees, cucurbits, etc.

It is well known that copper in too high a concentration is very toxic to most plant species, resulting in a reduction in general growth and the development of an induced iron deficiency. Interveinal chlorosis is very common and is often accompanied by other symptoms (reduction in the number of branches, darkening of the roots, etc.), which are aggravated in acidic soil.

Copper is not biodegradable and its spectrum on bio-aggressors is very broad, which explains two major drawbacks to its use:

• It accumulates in the soil and each new application increases its toxicity. Excessive copper content can even have an inverse effect on certain bio-aggressors such as Neofusicoccum Parvum in viticulture (3). Copper accumulation is so high in some vineyards or apple orchards that a return to market gardening is problematic.
• Its broad spectrum also affects useful soil microorganisms, leading to a reduction in biomass (4), a substantial alteration of the rhizosphere and a slowing down of rootlet formation, especially in acid soils. There is a disruption of iron migration leading to chlorotic-type phenomena. Zinc supply can also be disrupted, leading to a slowdown in growth. Thus, the excessive use of copper often found in organic farming is contrary to the conservation of soil biodiversity and disrupts ecosystem services; the very opposite of the fundamental objectives of this agricultural sector..

Copper-based preparations have been recognised as “of particular concern for public health or the environment” by the European Commission. The EFSA (European Food Safety Authority) and ECHA (European Chemicals Agency) considered that the use of these substances posed “certain risks to farmers, birds, mammals and soil organisms“. They are ‘responsible for the most common residues found in organic food‘. According to Didier Andrivon (INRA Research Director), “There are hundreds of studies showing that copper affects soil microbial communities and microfaunal components such as springtails” (5).

Other more recent studies on copper toxicity.

Epidemiological studies have shown that occupational exposure to certain metals, and in particular copper, is a risk factor for Parkinson’s and Alzheimer’s diseases (6 – 7). Moreover, a role for copper in prion diseases has also been proven (8), as recalled in a study published in July 2016 in the journal Science Advances, where American researchers confirmed how prions (proteins present in the brain) can become infectious in the presence of copper and cause spongiform encephalopathies. This discovery is reminiscent of the setbacks caused by rotenone, which was used in organic farming for many years before it was banned on suspicion of causing Parkinson’s disease.

And that’s not all. Bordeaux mixture triggers interstitial lung disease (sometimes fibrosis), characterised by the appearance of histiocytic granulomas and fibrohyaline nodules and also in some cases liver damage. A high incidence of adenocarcinoma has been reported in patients exposed to Bordeaux mixture (9-10). Copper has also been found in macrophages collected from the sputum of workers spraying Bordeaux mixture on grapes (11 – 12).

Copper is toxic to fish. It is classified as an undesirable substance in water intended for human consumption and in groundwater.

Copper accumulates more easily in acidic soil. Its toxicity is limited in calcareous and calcareous-clay soils, as copper tends to bind to carbonates and clays.

Repeated use of copper compounds can lead to resistance. In the southeastern United States, copper fungicides have been commonly used for decades in tomatoes to control Xanthomonas (Xanthomonas campestris pv. vesicatoria), a bacterial gall characterised by leaf spots. This bacterium has developed resistance to copper compounds (13) and researchers are now turning to genetics to find an alternative to copper compounds.

In the organic sector, whenever pesticides are tested in food, curiously, copper is never among the substances tested, probably in order to make people believe that organic farming is more respectful of the environment and of the health of consumers. In Nov. 2015, analyses commissioned by the website “agriculture et environnement” showed that 100% of the organic wines tested (29 wine samples from all regions of France) contained copper in proportions far higher than those of synthetic pesticides in conventional agriculture (14). The Esteban study (2014-2016) states on page 21 (15) that “Urinary copper concentrations are increased by 8% in children “consuming organically grown vegetables more than 4 times per week compared to those never or rarely consuming them.”

So should we abandon the use of copper in agriculture?

There is no agriculture without treatment, and there is no treatment without inconvenience. In organic or conventional farming, all farmers are obliged to control pests.

Of course, all the disadvantages of each treatment method must be compared. Each product has its qualities and defects, whether it is synthetic or natural. For example, in conventional apple growing, where many synthetic pesticides are used, some molecules (fenoxycarb, captan, boscalid, etc.) are considered probable carcinogens, and others are endocrine disruptors (fenoxycarb, fluopyram, etc.). These molecules also have a significant impact on the environment, although they are degraded by microflora (as well as their metabolites) over a more or less long period of time.

Since the ban on synthetic pesticides in France for the home gardener, copper is the only pesticide authorised to combat fungal and bacterial infections, although the risks of handling errors are known. This situation is all the more absurd when we know that the use of copper is more effective if it is used in alternation with other synthetic fungicides.

However, it is possible to reduce the use of copper against cryptogamic or bacterial infections of telluric origin (such as downy mildew in tomatoes) by a physical treatment using solar heating when climatic conditions allow it: solarisation, the technical principles of which are developed by clicking here.

In viticulture, downy mildew pressure can be reduced by using removable covers (viti-tunnels) that are deployed over the vines in case of rain.

The use of copper in conventional farming

In conventional agriculture, copper is still used preferably to control bean grease (a bacterial infection that attacks dry edible beans). There is no synthetic product registered for this disease. Copper is also used to control cercosporiosis (leaf diseases that mainly attack sugar beet, potato, bean, celery, melon, pepper and parsley), which is not always controlled by synthetic fungicides. On other crops such as grapes, copper is only a preventive measure.

In market gardening, Bordeaux mixture (copper sulphate) is often sprayed on young tomato plants in pots to prevent them from carrying fungal or bacterial diseases. Bordeaux mixture is also suitable for reducing the risk of alternaria on outdoor tomato crops, provided that you do not wait for the disease to develop.

Essential oils; an alternative to copper?

I have used essential oils several times to reduce the development of fungal diseases with mostly disappointing results. Some organic farmers or those specialising in permaculture or biodynamics claim to obtain acceptable results. However, a recent collective study conducted by INRA (b) and ITAB (c), among others, showed that although “the in vitro results show unambiguously that all the EOs tested have fungicidal activity, under field conditions (under shelter or in the open field), the project did not show significant efficacy at 0.2% (v/v) on the target fungi.” (16). This study aimed to find alternative solutions to the phytosanitary products used in conventional or organic agriculture. In this study, the essential oils tested alone or in a mixture were those most often used in organic agriculture (tea, clove, mint, thyme, savory, oregano, with or without an adjuvant) to combat fungal and bacterial diseases. Some preparations containing these essential oils are sold in permaculture farms.

This study also showed that some essential oils were toxic to bees, with varying degrees of mortality depending on the dose. The mixture of the 5 essential oils studied in this study was responsible for a mortality rate of over 80%. However, “At the lowest concentration of 0.2% (v/v), which is identical to that in the field trials, no toxic effects significantly greater than natural mortality were observed“. Laboratory studies suggest that essential oils may have a toxic impact on typhlodromes (natural predators of yellow and red mites and mites and erinose phytoptes).

a) Monosite action: action targeted on a single site of a cell, generally an enzyme essential to its metabolism.
b) INRA: Institut National de la Recherche Agronomique.
c) ITAB : Institut Technique d’Agriculture Biologique