Copper: The ancient metal with modern solutions in plant health

What do you have in mind when I mention the word “copper?” Perhaps the shiny metal that’s in our everyday life? Copper is one of the first metals humans ever extracted and used in large quantity. The first copper coins were minted around 8000 B.C. Copper tools are the defining features of the Bronze Age, starting about 3000 B.C.

For horticulturists and pest managers, copper is more than a useful and valuable metal. It’s an essential plant micronutrient and an effective bactericide, fungicide and algicide that has a place in every disease management and water treatment program. Some of the earliest fungicides are based on copper. Bordeaux mixture, which is prepared by mixing solutions of copper sulphate and quicklime, is credited with saving the French wine industry from downy mildew in the 19th century. It’s still used for management of potato blight, apple scab and other diseases.

Let’s raise a glass of wine, remember the contribution of copper to our society, and explore the many uses of copper in plant disease management and water treatment.

Mode of action and disease spectrum

Copper is classified in FRAC Group M1, which indicates its activity against multiple action sites within a pathogen. Specifically, copper ions disrupt, break down or inactivate proteins and enzymes in pathogen cells. Because of this multi-site activity, copper fungicides have a low risk of resistance development.

Having a low risk doesn’t mean having no risk at all. Resistance to copper has been demonstrated in several bacterial pathogen populations; therefore, rotation of multiple modes of action in a single crop should be practiced. On the flip side, the multi-site mode of action of copper fungicides makes them ideal rotation partners to other fungicides in every disease management program.

Copper products have been considered mainly for bacterial foliar disease management, such as pseudomonas leaf spots on hibiscus (Figure 1) and xanthomonas leaf spots on begonia. But copper is so much more than that! Copper is a broad-spectrum fungicide with activity against many common diseases (Table 1). Some of the most effective targets include powdery mildew, rust and algal and fungal leaf spots.

You may notice that the efficacy of copper fungicides varies greatly among diseases and even within a disease (Table 1). There are many factors contributing to variability, including experimental conditions, when the treatment was applied, pathogen species and even pathogen biotypes or isolates. The efficacy of copper fungicides can also be reduced in hard and highly alkaline water. The greatest efficacy of copper fungicides can be achieved through preventive treatment and shorter reapplication intervals.

Copper fungicides come in many forms

We now enjoy a variety of copper fungicides since the days of Bordeaux mixture. Copper fungicides registered for use on ornamental plants contain one of the following compounds: copper sulphate (basic copper sulphate and copper sulphate pentahydrate), copper hydroxide, copper or cuprous oxide, chelated compounds (such as copper octanoate and copper ethanolamine complex) and more. Depending on the compound used, each copper fungicide may have a unique metallic copper equivalent, signal word, restricted entry interval (REI) and OMRI status.

SePRO Corporation produces the widest array of copper fungicides, bactericides and algicides for terrestrial and aquatic uses. CuPRO 5000, Camelot O and Junction are available for management of bacterial, fungal and oomycete diseases on certain ornamental, vegetable, fruit and herb crops in fields, greenhouses and nurseries.

CuPRO 5000 contains copper hydroxide, a compound that has been in use since the 1960s. CuPRO 5000 provides the highest metallic copper equivalent (40%) among fungicides containing the same compound (such as 30% in Kalmor). The highly irregular and small particles in CuPRO 5000 allows for maximum release of copper ions, which results in superior efficacy. CuPRO 5000 is OMRI-listed and has the added benefits of moss and lichen management and frost injury protection.

Camelot O contains copper octanoate, or copper soap. The copper soap formulation is unique and has the benefit of achieving better spray coverage, which results in superior efficacy even at a low amount of copper applied. Camelot O leaves behind very little residue and is very safe for foliage and blooms. The safety of Camelot O is greatly appreciated in greenhouses, where the fear of phytotoxicity limits the use of copper fungicides. The REI of Camelot O is only four hours, allowing workers to return to work sooner. It’s OMRI-listed and can be used for organic vegetable and herb production.

Junction is a combination of copper hydroxide and mancozeb, which provides excellent broad-spectrum disease control. The premix also benefits resistance management by exposing pathogens to two compounds that challenge multiple activity sites. Junction comes in water-soluble packets, which makes the product safe and easy to handle. The formulation also allows easy mixing and application and achieves maximum leaf surface coverage.

Copper fungicides for water treatment

Many greenhouses and nurseries have one or more irrigation or retention ponds. Many ponds are often full of algae, which serve to support the growth of biofilms — a disgusting, slimy mix of algae, bacteria (such as erwinia or soft rot), fungi (such as rhizoctonia and fusarium), viruses, oomycetes (water molds such as pythium and phytophthora) and other pathogens. These biofilms can also grow on and clog irrigation heads.

The prevention of algal or biofilm growth in irrigation and retention ponds is the first step in managing many waterborne diseases if the pond water is recycled for irrigation. There are several “quick fixes” of algal problems in ponds. However, these solutions, such as hydrogen peroxide, sodium percarbonate/sodium carbonate peroxide and copper sulphate granules, often do not last long (thus constant reapplication is needed), are difficult to apply, require high amounts of products or special equipment and are often expensive.

SePRO Corporation provides multiple solutions for preventive and curative management of algae, biofilms and waterborne diseases. Visit bit.ly/avoid-algae to see how SePRO’s Aquashade, Captain XTR (copper ethanolamine complex), EutroSORB and SeClear (copper sulphate pentahydrate) can be combined in a program to manage algae and biofilms in ponds, irrigation heads and greenhouse and nursery floors.

Another valuable preventive tool is Cutrine Plus, which contains copper ethanolamine complex and is now labeled for management of oomycete pathogens in ponds and canals (Figure 2). Cutrine Plus can be used at a low rate (thus at lower cost), while still effectively reducing and preventing the establishment of oomycetes in the water column. Cutrine Plus is safe for plants and fish. Visit bit.ly/cutrine-plus to see how Cutrine Plus can be incorporated into your disease management program.

Copper is more than just a bactericide. It’s effective against other algal, fungal and oomycete pathogens that plague ornamental crops. It’s time for us to consider incorporating copper fungicides in all disease management programs.

Juang Horng (JC) Chong, Ph.D. is Technical Development Manager, Ornamental at SePRO. sepro.com/Hort