Recently I have been doing some research on the use of copper sprays in horticulture, specifically Avocado production in this case, but this post applies to most crops.I have had some disquiet about the use of copper for some years, but as I've learnt more about it, I've come to the point where I will no longer use it in my garden.
Copper has been used for more than 100 years as a fungicide, most famously in the Bordeaux mixture developed in 1882 in France to combat grape diseases. Copper is usually applied as either Copper Oxychloride or Copper Hydroxide, but is sold under many different brand names. Copper sprays are used as protection against the fungi that cause Downy mildew, Late Blight, Early Blight, Black Spot, Brown Rot, and various bacterial diseases. As it is a protective spray, repeated applications are sometimes needed to prevent the diseases becoming established.
Copper is a heavy metal that is also an essential trace element for both plants and most animals. It is found throughout nature and most soils have natural copper levels of 10-30 ppm, with sandy soils sometimes being deficient and some soils having natural levels up to 100ppm.
It is commonly accepted that copper sprays are organic sprays, which leads people to believe that they are safe to use. In fact copper sprays are some of the most toxic and persistent pesticides used in gardens and orchards. In recognition of this, international organic standards list copper as a restricted product. In New Zealand, Bio-Gro allows a maximum copper application of 3kg/ha/year. Copper fungicides for organic horticulture have been banned completely in the Netherlands and Scandinavia, and use has been restricted to 6kg/ha/year elemental copper in other EU countries since 2006. In gardening terms that’s roughly the equivalent of one or two wet-to-runoff sprays per year.
However, in non-organic production and in home gardens there are no such restrictions and indeed many garden writers and horticultural experts continue to promote the regular use of copper, often calling it organic. In some crops such as Avocados, growers apply up to 12 sprays per year, while home gardeners are sometimes told to spray roses, potatoes or tomatoes every 2 weeks during the growing season!
Copper fungicides are actually synthetic pesticides that disrupt and kill the cells of a very wide range of organisms. In humans, copper can cause problems such as liver disease and anaemia, but fairly high exposure to copper is needed to produce these effects in humans, so only horticultural workers or gardeners that spray copper without good protection need to be worried.
Copper sprays are of much more concern for the effect on soil life, particularly beneficial soil bacteria, fungi and earthworms. Soil copper levels as little as twice natural levels can reduce earthworm populations, while soils with levels of more than about 250ppm may have no earthworms at all. Vital nitrogen fixing bacteria are also inhibited once the soil level gets above this level.
Spraying with copper also kills beneficial microorganisms on the leaves and some studies have shown that some diseases can actually increase after copper sprays, as the beneficial microbes have been killed off, allowing the disease to flourish once the copper has been washed off by rain.
A typical copper spray can raise the topsoil concentration by up to 2 ppm, so with frequent applications toxic levels can be reached fairly quickly. You might relax at this point, thinking that the amount of copper you’ve applied would no way reach toxic levels. But do you know the history of your soil? Was it once a market garden or an orchard? Many suburbs and lifestyle blocks are established on old horticultural land. You might be adding copper to a soil that already has high levels.
Where copper sulfate has been regularly used over many years, such as the vineyards of Europe, soil copper concentrations can be found up to 1500 ppm. If this were an industrial site, it would be labeled as a toxic waste dump and shut down! In soils with a shorter history of regular copper use, such as Apple, Avocado and Stone fruit orchards, levels in the top layer of soil can reach up to 400 ppm, still high enough to drastically affect soil microbes and worms.
Copper does not degrade in the soil and is not easily leached out, except in very sandy soils, as it is bound onto organic materials, clay particles and mineral surfaces. Like the gift that keeps on giving, once the copper is in the soil, you are pretty much stuck with it. As it kills one microbe, which then rots away, the copper molecule is then available for the next microbe and so on.
Think carefully before using Copper sprays. Although they might be called organic, they may be doing more harm than good.