Copper-based pesticides are a group of many different compounds that have some form of copper in common as the active ingredient. These compounds have protectant activity against several bacterial, fungal and oomycete diseases. Although copper-based pesticides are one of the oldest class of fungicides (FRAC M1 fungicide), they are still used for management of many different diseases today. A combination of broad spectrum of activity, ability to withstand frequent wet weather events and inexpensive cost makes this group of compounds valuable in pest management programs. This overview of copper-based pesticides will cover Bordeaux mixture, copper sulfate along with many other forms of copper and resistance of some bacterial pathogens to these materials.
Bordeaux mixture, made by adding copper sulfate and calcium hydroxide to water, was one of the first fungicides ever used. Millardet, a French viticulturist during the mid-1800s, noticed less grape downy mildew on vines close to the road while walking through a vineyard one day (Klittich 2008). Using a brush, the grower had splashed a concoction from a bowl onto the outside row of grapes to prevent people from pilfering his grapes. (It gave the grapes an unappealing taste and look.) Within a few years, Millardet had perfected the formula and used a sprayer for application. This blue mixture is still used today to manage downy mildew in France and many other areas of the world.
Bordeaux has many positive and negative features. It is a highly effective bactericide and fungicide that is used to manage several plant diseases. The material sticks to and remains active on plant surfaces even during typical wet PNW winters. Generally it is used as a dormant spray because it may burn young juvenile tissues. The ingredients must be mixed in the right order and with mechanical agitation of the tank to avoid the formation of a sprayer clogging precipitate. Bordeaux cannot be mixed ahead of use because it deteriorates on standing.
Many other copper-based pesticides have been developed to capture the positive weathering and disease management features of Bordeaux without the challenges of preparing the material properly. Copper-based active ingredients in other products include copper ammonium complex (Copper Count-N), copper hydroxide (Champion, Kocide, Nu-Cop, etc.), copper oxide (Nordox), copper oxychloride, (C-O-C-S), and copper sulfate (Cuprofix Disperss, many others).
The active ingredient in all copper-based formulations is the positively charged copper ion (Cu+2). Many organisms are sensitive to very small amounts of copper ion, such as bacteria and fungi but especially aquatic organisms such as algae or water molds (including oomycete pathogens like downy mildews). Copper-based products have broad-spectrum activity against microorganisms due to copper’s interaction with nucleic acids, interference with energy transport, and disruption of enzyme activity and integrity of cell membranes.
The same small amounts of copper are not toxic to plants or humans. Copper is important for the formation of red blood cells, activity of antioxidant enzymes and assists with the formation and maintenance of connective tissues in the human body (Higdon et al 2013). Although copper is widely distributed in the foods we eat, copper toxicity is rare.
Copper at moderate to high doses may be toxic to plants. Bluestone which is copper sulfate pentahydrate (CuSO2) disassociates in water releasing a high concentration of copper ions. This high concentration of copper ions is toxic to actively growing plants and washes off quickly with rain. Mixing hydrated lime with copper sulfate ties up or “fixes” most of the copper ions, which makes the mixture safe (safened) for application to plants. Other forms of copper used for plant disease management, such as copper hydroxide, copper oxide, copper oxychloride and copper octanoate, are formulated to produce low doses of copper to reduce toxicity to plants. The goal of most copper-based products is to tie up or fix much of the free copper ions so it is not phytotoxic to plants but allow just enough of the copper ions to be released to inhibit disease causing pathogens.
Different copper formulations will result in different amounts of copper ion released. Most products express the amount of copper they contain in terms of copper metallic equivalents. Unfortunately, the copper equivalent does not directly relate to the amount of copper ion released. Scheck and Pscheidt 1998 found several formulations of 50% metallic copper produced a wide range of copper ion in solution.
Acidic conditions result in a higher concentration of copper ion. When copper-based pesticides are tank mixed with acidic compounds more copper ion may be released, which can lead to phytotoxicity. Read label warnings carefully to avoid these situations.
Fixed Copper Sulfate (CS 2005, Cuprofix, Mastercop, Instill, Phyton and many others)
Several products contain a fixed copper sulfate and may list the active ingredient as basic copper sulfate (Cuprofix) or copper sulfate pentahydrate (CS 2005, Mastercop, Instill, Phyton 27). These products can be used on a wide variety of crops to manage bacterial, and fungal and oomycete diseases. Phytotoxicity can still occur with some crops such as fruit marking of cherry and russeting of some pears. To prevent fruit marking of cherry, applications are only allowed during and prior to bloom or after harvest. Good (fast) drying conditions are important to avoid fruit russet risk. Read label warnings carefully to avoid damage to crops.
Some of these copper-based products claim to be “systemic” implying that when sprayed onto one part of the plant, the copper can move internally to other parts of the plant for better disease control. This activity has never been proven (Lamichhane et al 2018). Increased copper uptake and stem concentrations of copper was shown to occur in tobacco plants treated with copper sulfate-amended water (Ge et al. 2020). The concentration of copper was so low that it did not inhibit the xylem inhabiting pathogenic bacteria Xylella fastidiosa. The copper concentration increased the bacteria’s growth by stimulating biofilm formation. There are plants that naturally have high leaf or stem copper concentrations when growing in high soil copper conditions (Lange et al. 2017). The root to shoot translation factor, however, is very low. Many of these are weedy plants found growing in extreme and or tropical environments. Several are also used for medicinal purposes in traditional cultures such as a widely cultivated herb from India, Ocimum tenuiflorum, as well as Waltheria indica and Clerodendrum infortunatum.
Some products may have a trade name that sounds or looks like Bordeaux such as “Bordeux” but this product actually contains tribasic copper sulfate.
Copper Hydroxide (Champion, Kocide, Nu-Cop, Previsto and many others)
Chemical analysis of Bordeaux mixture found that one of the products produced was copper hydroxide (Lamichhane et al 2018). Many products contain the active ingredient copper hydroxide including Champion, Kocide, Nu-Cop, Previsto and many others. These formulated materials are about as effective for disease management as Bordeaux without the mixing problems.
Formulations of copper hydroxide can vary considerably in the amount of free copper ion found in solution and the degree of effective disease management (Scheck and Pscheidt 1998). Use of a liquid formulation of copper hydroxide (Kocide LF) resulted in fewer free copper ions and more bacterial blight on lilacs than dry formulations (Kocide DF or 101). In general, dry formulations of copper-based pesticides have resulted in better fungal and bacterial disease management than liquid formulations in trials conducted in western Oregon.
A formulation of copper hydroxide suspended in an alginate matrix (Previsto) has been effective against several diseases such as fire blight of pome fruit while using less overall copper. Pome fruit russeting can occur with applications of copper-based bactericides, but less russet was reported with Previsto used in arid production areas.
Copper Oxide (Nordox)
The red material cuprous oxide was first used as a seed treatment and later as a foliar fungicide. Formulated as Nordox the label allows application to a wide variety of crops for management of many different diseases. Cuprous oxide also has similar precautions as the fixed copper sulfates with pH and copper sensitive crops to prevent phytotoxicity. Rarely tested in the PNW, but has been used to significantly enhance pea emergence as a seed treatment. Clear conclusions about usefulness could not made due to variable results from year to year and no control seen in root rot evaluations. A few trials in California found 50% control of peach leaf curl and grape powdery mildew using Nordox.
Copper Octanoate (Cueva)
Copper mixed with naturally occurring fatty acids forms copper salt of fatty acids, technically known as soap with an overall lower concentration of copper. Cueva contains copper octanoate, which is a blue material that can be used in the organic production of many crops. The lower copper ion release helped reduce phytotoxicity (russeting) in fire blight control trials in semi-arid Washington but have shown some risk of russeting in wetter areas of Oregon and California. West coast trials for management of spinach downy mildew, grape powdery mildew, or apple scab averaged about 50% control with variable results from year to year. Cueva was not effective against the fungal disease eastern filbert blight while other copper products generally resulted in good control.
Copper Ammonium Carbonate (Copper-Count-N)
The green precipitate copper ammonium carbonate occurs when sodium carbonate is added to a copper sulfate solution. When sprayed onto plants the ammonia evaporates leaving a long lasting residue of copper carbonate, basic copper sulfate and/or copper hydroxide (Thomson 2000). Formulated as Copper-Count-N, the label allows application to a wide variety of crops for management of many different diseases. The label lists precautions similar to those of other copper-based pesticides to prevent phytotoxicity with pH and copper sensitive crops. This product has not been evaluated in a lot of trials on the west coast. Studies in California found Copper-Count-N resulted in 72% control of the bacterial disease walnut blight. Additionally, excellent control of olive leaf spot was reported but copper in any formulation also resulted in excellent control when disease pressure was low (Teviotdale et al 1989).
Copper Oxychloride and Combinations – (Badge, C-O-C-S)
Copper oxychloride is a green to blue-green compound used for disease management. Not available alone but often mixed with other copper-based materials. The product C-O-C-S is composed of both copper oxychloride and basic copper sulfate. Badge is composed of both copper oxychloride and copper hydroxide. Both products have the same warnings about pH and copper sensitive crops.
In lilac tissue culture, C-O-C-S resulted in 80% control of copper sensitive isolates of Pseudomonas syringae but only 27% control of copper resistant bacteria (Scheck and Pscheidt 1998). C-O-C-S also was excellent in the same olive leaf spot trial mentioned above where all copper-based products did well.
Data on the use of Badge in west coast trials is still limited but in general control ranged from 53% to 92% for diseases such as peach shothole (92%), peach leaf curl (50%), grape powdery mildew (66%) and eastern filbert blight (53%).
With good application coverage, Bordeaux sticks well to plant surfaces and does not need additional adjuvants added to the mix. Other copper-based pesticides, however, have determined that spreaders and/or stickers added to the tank mix will improve product performance. For example, many copper-based products suggest to add a superior-type oil to help break the surface tension of water for better coverage on the target crop. Again, read labels and follow manufacturer recommendations about using additives with these products.
Without adjuvants, persistence of copper hydroxide (Kocide 2000), cuprous oxide (Nordox) and copper oxychloride sprayed separately onto orange trees was similar (Schutte et al 2012). Residues of each product decreased fast during the first 14 days followed by a more gradual decline over the next 6 weeks. The loss of copper residues was attributed to weathering (days after treatment), fruit growth and rainfall.
Bacterial resistance to copper-based products has been documented and limits their usefulness (Lamichhane et al 2018, Renick et al 2008, Scheck and Pscheidt 1998). Pseudomonas syringae isolates resistant to copper have been found on many crops in the PNW including fruits such as blueberry and pear and ornamentals such as Euonymus, Forsythia, lilac, maple, Mock orange, Mountain ash, Oregon grape, Otto Lueken, Sweet gum (Liquidambar sp.), and Viburnum sp. (Scheck et al 1996, Spotts and Cervantes 1995, Stockwell et al 2015). Xanthomonas spp. resistant to copper have been found in North America on citrus, tomato and walnut (Lamichhane et al 2018, Ninot et al 2002). Isolates of Erwinia amylovora with a slightly increased tolerance of copper were reported in British Columbia (Sholberg et al 2001) but it was expected that copper-based products would still be effective against fire blight. At the extreme are copper resistant bacteria (Bacillus sp. strain 105) that thrive in copper mines in Brazil converting copper sulfate into metallic (zero valent) copper (Gracioso et al 2021).
In an interesting side note, there were very few mentions of fungal resistance to copper-based products in a book devoted to fungicide resistance. There was only a passing comment about copper tolerant Botrytis (Stevenson et al 2019).
Resistance to copper-based pesticides may be discovered through disease management failures. Management failures, however, may be due to many other reasons including poor coverage, inadequate timing, low application rate, reinfection from outside sources or even systemic infections. Use of copper-based bactericides also has been known to increase disease as in the case of bacterial canker of sweet cherry (Pscheidt and Cacka 2009).
ManKocide and Junction
A slight increase in copper ion concentration may be enough to manage some bacterial diseases where the pathogen has developed resistance to copper (Lee et al 1993). Improved control of bacterial spot of tomato, halo blight of beans, or walnut blight have been reported from applying a mixture of copper hydroxide and the fungicides maneb or mancozeb (Conover and Gerhold 1981, Teviotdale et al 2002, Zhang et al 2017). This mixture produces a copper carbamate, which is more effective than the copper hydroxide alone. Unfortunately, bacterial pathogens may adapt to the new, increased concentration of copper ion generated with this strategy.
Some products come pre-mixed with a combination of copper hydroxide and mancozeb. These products include ManKocide for agricultural and seed treatment use and Junction for ornamental and turf use. These products can be used on fewer crops, due to the restrictions on mancozeb rather than the copper hydroxide.
After decades of using copper-based pesticides in Europe, copper accumulation in the soil has become a major concern (Lamichhane et al 2018). The soils are considered polluted with copper ions that have bound to soils after washing off of crops. The ions are bound to particles of organic matter, clay, and metal hydroxides. Downward movement of copper through the soil profile is greater in sandy soils than soils rich in clay or organic matter. Copper availability and toxicity in the soil is increased as the soil pH decreases. Restrictions on copper-based pesticide use have been implemented in many areas of the world and is advisable for the PNW.
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