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2020. July 28. News

With or without copper?

First results of the RELACS project experiments in Hungary on substituting copper in plant protection.

Since 2019 the Hungarian Research Institute of Organic Agriculture (ÖMKi) coordinates a series of experiments with the aim to test the experimental pesticide BPA038F. This pesticide contains tagatose and seems suitable to protect plants against grape downy mildew (Plasmopara viticola). To understand the significance of copper usage and how it may be reduced, it is important to overview the role of copper in plant nutrition, its significance in plant protection and its environmental effects.

Copper as a nutrient for plants

Copper is an essential micronutrient for plants. It can be found in many important enzymes and it also plays a fundamental role in chlorophyll production. Some cultures, like cereals for example, have higher copper requirements than others. If the supply of copper is insufficient, the leaf tips of cereals become bleached. This is followed by the formation of narrow, curled, twisted leaves and incomplete or empty ears (Loch and Nosticzius, 2004). Copper insufficiency rarely occurs in practice, but commonly develops in soils that are rich in humus, in which case the insufficiency is not due to the lack of copper but its bioavailability.


A copper-insufficient wheat ear (source: International Maize and Wheat Improvement Center, 2006)

Copper in the soil

Copper naturally enters the soils from the rocks during soil formation. As a result of this process, the tilled layer of cultivated lands in Hungary has an average copper content of 5.4 mg per kg (Kádár, 1998). However, heavy metals such as copper can get into soils via human activity also. Waste incineration, metallurgy and mining of non-ferrous metals, as well as agricultural activities (organic and synthetic fertilizers, pesticides and sewage sludge) all present an environmental load.

Copper has poor mobility in the soil as it is mostly bound to organic and inorganic adsorptive surfaces, and in excess concentrations, it can even prevent the uptake of iron, manganese and zinc (Loch and Nosticzius, 2004). In addition, plants take up only a fairly small amount of copper, which may lead to an increase of soil copper concentrations over time when the load of copper is continuous. Certain factors such as wind and water erosion may mitigate the rate of accumulation, but this does lead to the complete elimination of copper. Instead, it can make its way to other areas or into water bodies.

Copper has an impact on several members of the soil biota. It has been known for a long time, that earthworms prefer to live in soils with lower copper contamination and their reproductivity also increases in such areas compared to areas with higher copper content (Ma, 1988). Earthworm migration from contaminated areas increases and the possibility of recolonization decreases. Because of this, the soil enhancing activities of earthworms have less of an effect in soils with high copper concentrations, which in turn has a negative impact on water management of soils.

A study carried out by Szabó (2017) sheds light on the average copper concentration of soils in Hungarian wine regions. Szabó (2017) studied 247 vineyards in the 0-30 cm and 30-60 cm layers of the soil. Table 1. shows that in certain wine regions, copper concentrations are up to six times (in 0-30 cm depth at the Tokaj wine region: 34.36 mg per kg) of the average of cultivated lands in Hungary (5.4 mg per kg). The highest copper concentration was measured in the old, traditional wine regions of Hungary.

Data indicates the potential of copper to accumulate in soils when it is being used as a pesticide, and how such agricultural activities may result in significant, long-term soil contamination.

.

 

Average Cu (EDTA) mg per kg

Sample size

Wine region

0-30 cm

30-60 cm

0-30 cm

30-60 cm

Badacsony

10.97

3.64

45

43

Balaton-felvidék, Csopak

5.39

4.48

13

13

Eger

16.58

13.71

68

68

Kunság

19.08

16.22

32

32

Mátra

6.33

3.61

6

6

Sopron

22.22

20.31

5

5

Tokaj

34.36

30.43

37

37

Tolna

17.93

6.70

3

3

Villány

15.73

12.53

35

35

Table 1: Average copper concentrations in the soils of vineyards in various major Hungarian wine producing regions, at various depths, with the number of assessed plantations (source: Szabó, 2017)

Copper as a pesticide

The use of copper in plant protection began several hundred years ago. Sources indicate that in 1761 a diluted copper sulphate solution was already used to decrease damage caused by seed-borne diseases to immersed beans. However, the real breakthrough in terms of the use of copper came in 1885, when French botanist and mycologist Pierre-Marie-Alexis Millardet proved that copper was able to combat downy mildew. Millardet was the first to observe that the rows of the vineyards in Bordeaux, treated with copper to deter thieves and herbivores, were noticeably less infected with grape downy mildew. Based on his observations, he developed an aqueous solution containing a mixture of copper sulphate and slaked lime - later became known as “the Bordeaux mixture” - which is still a major asset in the protection of grapes and fruit-bearing plants to this day (Günther, 1998).

The Bordeaux mixture has antibacterial and antifungal effects due to the free copper ions (Cu+, Cu2+) in it, which may be present in the spraying mixture or may be released from copper crystals after being exposed to organic compounds produced by fungi. When copper ions enter the cells of a pathogen, they can bind to many different chemical groups, which impairs the function of enzymes and other proteins. This results in cell damage and leaky membranes, and finally the death of the pathogen (Husak, 2015).

As a pesticide, copper has many favourable characteristics. It is effective at low ambient temperatures and it is suitable for fighting many bacterial and fungal species. While in most cases, modern, absorbable substances act via a given biochemical process (e.g. they inhibit the functioning of a certain enzyme), copper can antagonize the physiological processes of pathogens in several ways. One of copper’s great advantages is that pathogens cannot become resistant to it, which is certainly not true of modern substances with targeted mechanisms of action.

A disadvantage of antifungal products containing copper is that in order to ensure their efficacy, a film has to be formed and maintained on the surfaces of the plants, as they cannot enter the plant tissues. Hence, they can only provide a preventive protecting effect against bacterial and fungal infections on the surfaces of the leaves. They also dissolve easily in water, which promotes wash-off. Because of this, it is important to ensure proper coverage and for effective protection, it is also essential that the active substance stays – as far as possible – at the site of the application.

It is also important to note that stone fruit crops are sensitive to copper when blooming and copper can also cause cosmetic damage to certain types of apple (Ábrahám et al., 2011). With grapes, pesticides that contain copper during early intense shoot growth can shock the development of green parts and roots, resulting in poor growth of the plant (Hajdu, 2011). In addition, according to the Commission Implementing Regulation (EU) 2018/1981, the seven-year-average annual amount of metallic copper applied on one hectare should not exceed 28 kg, in other words, a maximum of 4 kg per year. As there are far fewer pesticides available for organic agriculture to prevent fungal and bacterial diseases, this sector is more dependent on pesticides that do contain copper. In order to reduce this dependency, new, alternative pesticide active substances are needed.

The origin and uses of a promising alternative, D-tagatose

D-tagatose is a natural sugar with a molecular formula identical to that of glucose (C6H12O6), and its structure is the mirror image (enantiomer) of that of fructose (see: Figure 1). It can be found in the exudate of a tropical tree, Sterculia setigera, and in certain lichen species (Rocella spp.) as the building block of specific oligosaccharides. Through the enzymatic processes of various bacteria, galactose is partly metabolized into D-tagatose. It can also be found in heat-treated dairy products, as lactose is also transformed into D-tagatose in small amounts when exposed to heat. The D-tagatose concentration of sterilized cow’s milk and milk powder varies between 2 and 800 ppm, but it also occurs in other dairy products, including yoghurt (Bär, 2004).

It is most commonly used in cereal flakes, fizzy drinks, pastries, ice creams and chewing gum as a low-calorie sweetener. It is also used to mask unpleasant flavours in medicines. Based on studies carried out by the US Food and Drug Administration (FDA) no long-term toxic effects can be expected in humans even when consuming tagatose in large amounts (Ibrahim, 2018). 

D-tagatose as an active substance for pesticides

Based on observations, some “rare sugars” (i.e. sugar compounds that rarely occur in nature or only in small amounts) such as D-allose or D-psicose induce systemic acquired resistance (SAR) in certain plants, which is a non-specific defence system of plants. This results in increased resistance against many types of pathogens. The process starts with the induction of resistance genes, which is also activated by D-tagatose, but this does not always manifest in an actual plant protecting effect. However, its efficiency was confirmed in downy mildew of cucumber, cabbage and grape, damping-off caused by Phytium species, the Phytophthora disease of tomato and potato, and the powdery mildew and stem rust diseases of cucumber and barley (Ohara et al., 2008). However, the direct effect of D-tagatose on these pathogens has not yet been clarified (ibid.).

In vitro tests showed that the growth of the pathogen causing late blight (Phytophtora infestans) is limited on broth treated with tagatose (Chahed, 2020). Investigations carried out caused severe ultrastructural alterations, with the formation of circular and concentric mitochondrial cristae of the pathogens as well (ibid.). These observations indicate that tagatose is worth investigating under field conditions for its plant protecting effects, as we can suppose that it has a direct effect on certain pathogens.

Figure 1: The chemical structure of D-tagatose (source: georganics.sk)

Application of tagatose in a Hungarian vineyard

In 2019, we were the first in Hungary to test the experimental product called BPA038F, containing tagatose as active substance, in field conditions. The trial was undertaken in the frame of the project RELACS (Replacement of contentious inputs in organic farming systems). We tested the product in the organic vineyard using the Kékfrankos variety (also known as Blaufränkisch) In this experiment, we compared the efficiency of two different treatments to prevent downy mildew of grape. One treatment comprised of products used in organic viticulture practices: a combination of Champion WG (copper-hydroxide) and Microthiol Special (sulfur). The other treatment also included Microthiol, but in this case, 1% BPA038F was added to the mixture instead of the copper-containing product.

The experimental area included eight rows, and treatment types were applied in four rows each. We assessed the 50 vines in each row at three different time points: at the end of the blooming period (BBCH 71-72), at the beginning of skin coloration of berries (BBCH 80-81), and at veraison (BBCH 85-75). 

At the first assessment grape downy mildew was more prevalent on the foliage that had been treated with tagatose compared to the copper-treated rows, although this difference was not significant. However, it was clear that the majority of downy mildew spots seemed to be drying in the tagatose-treated rows, (Figure 2) while although the number of oil spots was lower on the copper-treated plants, the pathogen was still actively producing spores. The severity of the infection was similar in tagatose-treated and copper-treated rows. No bunches infected with downy mildew were present in any of the rows.


Figure 2: Dried spots of downy mildew (photograph: Máté Varga, 2019)

At the second assessment, spots of downy mildew of grape were still more common on the foliage treated with tagatose than those treated with copper. By this point, a significant number of dried spots of downy mildew were present on foliage treated with both substances. The severity of the infection was similar on tagatose-treated and  on copper-treated rows. By this time, powdery mildew of grape (Erysiphe necator) had also appeared in the culture. Active powdery mildew was observed in a spot-like pattern on bunches more covered by foliage (Figure 3), and a negligible number of berries had already started splitting open. The symptoms of powdery mildew had appeared on older leaves too, but there was no difference between the two treatments regarding its occurrence.


Figure 3: Active spots of downy mildew on the face of a leaf (photograph: Bence Trugly, 2019)

At the third assessment, downy mildew was less common on vines treated with tagatose compared to plots treated with copper. This difference was statistically significant. However, the majority of spots seemed to dry with both treatments. No symptoms of downy mildew occurred on the bunches with either treatment. Powdery mildew of grape was still present in the culture (Figure 4 and 5).


Figure 4: Powdery mildew of grape on berries in veraison (photograph: Bence Trugly, 2019)


Figure 5: Signs of juvenile bunch powdery mildew on berries that have started to ripen (photograph: Máté Varga, 2019)


Figure 6: Berries starting to ripen (photograph: Máté Varga, 2019)

Summary

Due to the high amount of precipitation in May in 2019, downy mildew of grape became a serious plant protection challenge in most Hungarian wine regions, however due to the orientation of the vineyard involved in the study and the appropriate agrotechnical measures, the pathogen could be controlled with five copper or tagatose treatments. In the copper-treated rows, the infection was significantly more common at the last assessment than in tagatose-treated rows. The severity of the infection on foliage treated with tagatose was less than in the case of the rows which was treated with copper, although this difference was very small. While this difference is not significant, it is still a surprising finding, as the studied product was able to achieve the same efficiency as the control copper treatment. This experiment will continue in 2020 and 2021, and we will also continue to investigate the product.

Acknowledgments

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 773431 (project RELACS).

References

Commission Implementing Regulation (EU) 2018/1981 of 13 December 2018 renewing the approval of the active substances copper compounds, as candidates for substitution, in accordance with Regulation (EC) No 1107/2009 of the European Parliament and of the Council concerning the placing of plant protection products on the market, and amending the Annex to Commission Implementing Regulation (EU) No 540/2011 Published: 13 December 2018

Ábrahám, R., Érsek, T., Kuroli, G., Németh, L., Reisinger, P., (2011) Növényvédelem. (Plant Protection) p. 73

Bär, A. (2004). D-tagatose, dossier prepared and submitted by Service. Bioresco on behald of Arla Food

Chahed A., Nesler A., Navazio L., Baldan B., Busato I., Ait Barka E., Pertot .I, Puopolo G. and Perazzolli M. (2020). The Rare Sugar Tagatose Differentially Inhibits the Growth of Phytophthora infestans and  Phytophthora cinnamomi by Interfering With Mitochondrial Processes. Front. Microbiol. 11:128.

Günther, J. (1998). Copper: Its Trade, Manufacture, Use, and Environmental Status. p. 368.

Hajdu, E. (2011). Szőlőtermesztésben előforduló fejlődési zavarok és gyógyításuk (Developmental disorders in viticulture and their treament) in: Terbe, I., Slezák, K., Kappel, K. (2011). Kertészeti és szántóföldi növények fejlődési rendellenességei (Developmental disorders of horticultural and field crops) p. 209.

Husak, Viktor. (2015). COPPER AND COPPER-CONTAINING PESTICIDES: METABOLISM, TOXICITY AND OXIDATIVE STRESS. Journal of Vasyl Stefanyk Precarpathian National University.

Ibrahim, O. (2018). A New Low Calorie Sweetener D-Tagatose from Lactose in Cheese Whey as a Nutraceutical Value-Added Product.

Kádár, I. (1998). Kármentesítési Kézikönyv. (Remediation Manual) XII. Talajszennyezettség minősítése a hazai szabályozásban. (Assessment of soil contamination in the Hungarian regulatory environment) Ministry of the Environment, Budapest.           http://fava.hu/kvvm/www.kvvm.hu/szakmai/karmentes/kiadvanyok/karmkezikk2/2-13.htm

Loch, J., Nosticzius, Á. (2004). Agrokémia és növényvédelmi kémia (Agro-chemistry and plant protection chemistry) p. 93-94.

Ma, W. (1988). Toxicity of Copper to Lumbricid Earthworms in Sandy Agricultural Soils Amended with Cu-Enriched Organic Waste Materials. Ecological Bulletins, (39), p. 53-56.

Szabó, Á. (2017). Magyarországi szőlőültetvények talajának rézszennyezettsége. (Copper contamination levels in the soils of vineyards in Hungary) Thesis. Szent István University, Gödöllő.


Replacement of Contentious Inputs in Organic Farming Systems’ (RELACS) has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 773431. The information contained in this communication only reflects the author’s view.

 

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