In gardening circles, some practices have been in long use without a lot of scrutiny as to their effectiveness or side effects. The use of copper sprays is one of these; quite effective as protection against the fungi that cause downy mildew, late blight, early blight, black spot, brown rot, and various bacterial diseases, but with side-effects that most people are unaware of.
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. It is a short-lived protective spray, so repeated applications are sometimes needed to prevent diseases becoming established.
Copper is both a heavy metal and an essential trace element for plants and animals. It is found throughout nature and most soils have natural copper levels of 10-30 parts per million (ppm) with 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 or sometimes even a banned product.
However, for home gardens there are no such restrictions and gardeners might spray roses, potatoes or tomatoes every two weeks during the growing season and undertake winter sprays for pipfruit and stonefruit. Subtropicals, such as avocados, citrus and passionfruit, also routinely get copper sprays.
Copper fungicides are synthetic pesticides that disrupt and kill the cells of a 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 also very toxic to beneficial soil bacteria, fungi and earthworms. Soil copper levels as little 60ppm 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 copper level gets above this level.
A typical copper spray can raise the topsoil concentration by up to 2ppm, which on low copper soils might not be a problem. However, many suburbs and lifestyle blocks throughout New Zealand 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 decades, such as the vineyards of Europe, soil copper concentrations can be found up to 1500ppm! In soils with a shorter history of regular copper use, levels in the top layer of soil can reach up to 400ppm – still high enough to drastically affect soil microbes and worms.
Copper does not degrade and leaches slowly from soil, as it is bound on to 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!