That's because in order for organic agriculture to exist, the thought is that copper fungicide is the key to getting reliable harvests...but on the other hand, there are known environmental impacts surrounding the use and application of copper.
I think that frequently, because it is known the copper is allowed in organic agriculture, that home gardeners apply it without fully realizing the implications of its usage.
In this article, I am going to outline why copper products are applied in farms and gardens, the scientific data on the environmental effects, and the potential alternatives for copper in your home garden.
Let's have a look at the facts so that you can decide for yourself if and how you want to apply copper products to your home garden.
Copper fungicide is often applied to fight off foliar fungal and bacterial issues that can have detrimental impact on crop yield. Arbico lists that their copper product can be sprayed for:
On the Arbico Organics website, their copper fungicide is made from copper sulfate and lime and is for home gardens only. Arbico explains that the copper "damages enzymes and proteins critical to fungal growth."
One of the more popular formulations of copper fungicide in the agriculture industry is called a "Bordeaux mixture," a combination of copper sulfate with lime (similar to the Arbico product mentioned above).
University of Cailifornia's Integrated Pest Management Program explains that Bordeaux mixture is especially helpful because in addition to its fungicidal properties, it is also a bactericide, which prevents issues such as bacterial leaf spots and fire blight. Applications of this mixture are able to withstand rain and so do not need to be reapplied as frequently.
Otherwise, copper-based products typically need to be applied repeatedly, either due to rain or for the targeted disease to be fully eradicated. Copper can accumulate in the soil, which can negatively impact plant growth in future years. Plants such as legumes, cereals, and grapes are known to be especially effected by copper in the soil (inrae).
Yes, copper is important for plant health. The University of Minnesota Extension shares that copper supports plants in making chlorophyll and set seeds.
However, there is a range of copper that is beneficial to plants. If plants are exposed to too much copper, they can show signs of copper toxicity.
If too much copper is present in the soil, there may be poor germination, a lack of proper root formation, and a lack of resiliency and health in the plant.
The Australian Institute of Horticulture explains that since copper is a heavy metal, it is very slow to break down once it is in the soil, which is why it is so easy to have it accumulate.
Rutgers University states that soil is considered copper deficient if a soil test result is less than 5ppm, but copper levels of 20ppm and higher should be monitored.
I found this piece of information to be extremely interesting: Rutgers states that "The key with copper is that it must be applied before the presence of disease in order to be effective."
I think sometimes gardeners see the foliar disease issues, and then spray copper to help manage it. This piece of information may help home gardeners to make more informed decisions about what they apply to their garden and when.
Arbico Organics, who makes a copper fungicide approved for home organic use, shares in the technical details that you should wear long sleeves and pants, goggles, a mask, and gloves, all made from water proof materials, when applying the product. It is then recommended to immediately change and wash all items that you were wearing.
Of course, if you are going to apply copper please thoroughly read the instructions and follow the manufacturer's directions for application.
Soil microbes do not fair well when copper products are applied, as the copper products to not discriminate between beneficial and non-beneficial fungi and bacteria.
This report from the Australian Society of Soil Science shares "Effects can occur at relatively low Cu concentrations and influence a number of soil processes including microbial activity, earthworm activity and bioturbation. In most soils, copper residues are likely to remain indefinitely, and will continue to influence the health of the soil."
Another source that speaks to the low levels of copper needed for a negative impact on the soil shares that even small increases of copper in the soil decrease the population of beneficial mycorrhizae and thus decreases their beneficial relationship with plants.
Worms, the gardener’s best friend, are also shown to have a higher die-off rate on fields applied with copper products (inrae).
A report from the USDA shares this striking quote: "Most animal life in soil, including large earthworms, have been eliminated by the extensive use of copper-containing fungicides in orchards"
Since pathogenic fungi are adaptable, the Australian Horticultural Society notes that they can become resistant to the copper applications, thus perpetuating the cycle of reapplication.
Regenerative agriculture and gardening practices prioritize the health of the soil above all else: we are growing soil microbe populations, increasing photosynthesis, and supporting carbon sequestration in the soil first and foremost. So while blight and mildew may be causing damage to your crops, you may want to examine if copper is the best solution for your overall goals if you are implementing regenerative gardening in your situation.
A review of the use of copper fungicide in organic agriculture by the USDA states that "Bees are endangered by Bordeaux mixture. Copper sulfate may be poisonous to sheep and chickens at normal application rates." The Australian Horticultural Society cautions "Spraying fungicides when pollinators are present is counterproductive and environmentally irresponsible."
If you are someone who plants a bee garden or pollinator patch but also sprays with copper, this is something to consider.
If you're a regenerative farmer rotating chickens or sheep into your orchards or agroforestry systems, be cautious of spraying copper for the health of your sheep.
Arbico Organics states in their Copper Fungicide info sheet that it is "toxic to fish and aquatic invertebrates and may contaminate water through runoff."
A study from 2022 found that when copper interacts with naturally occurring carbon in the soil, that the combination creates 2 gases which are known to damage the ozone layer: bromine and chlorine.
This study calculates that based on the amount of copper applied to the soil in agriculture each year, 6,500 metric tons of ozone destroying chemicals are released into the atmosphere.
As regenerative gardeners and farmers, one of our main guidelines is to prevent further harm from happening to the environment. This new information on copper fungicides contributing to the depletion of the ozone layer is one additional reason for me personally to not use organic-approved copper on my plants.
As a fungicide, it makes sense that copper could also harm mycorrhizal fungi.
The problem with this is that mycorrhizal fungi form beneficial relationships with plants, and help sequester carbon in the soil.
A recent study published in Rhizosphere Journal found a linear correlation between copper and mycorrhizal networks. The more copper that was applied, the less mycorrhizal populations existed.
What's interesting is that farmers noted a correlation between copper applications and a decrease in tree health in the 50's on citrus orchards. Then In the 80’s, Graham et al found the tree issues could be attributed to the loss of the plant-fungal benefical relationship. Since mycorrhizae deliver essential nutrients like phosphorus and nitrogen to plants, this relationship was no longer robust (Hartnett).
It's becoming more and more clear that mycorrhizal relationships in the soil need to be nurtured for the health of plants and the planet. The less mycorrhizal relationships happening, the less that carbon sequestration can occur in the soil. Mycorrhizal fungi are a nature-based solution to climate change that should be encouraged instead of harmed.
The report from the Australian Society of Soil Science and the Australian Institute of Horticulture referenced above highlights the following potential alternatives to copper:
I will also look at the solutions to fungal and bacterial issues that are outlined in JADAM practices.
Let's look at what is known about the effectiveness of each of these copper alternatives.
While this study was done on rice sheath blight, the findings are promising. A mature compost reduced rice sheath blight by a range of 32%-39%.
The AIH notes that compost teas can help prevent fungal disease from setting in on plant leaves. This is because the "good" bacteria in the compost tea will be established first, and the pathogens will not be able to take over.
A study out of Woods End Research Laboratory looked at how compost applications could reduce incidence of Powdery Mildew in grape crops.
They found that cattle manure composts and horse manure composts can reduce levels of the bacteria that causes powdery mildew. They reason that the cause for this is the high level of microbes within the compost as preventing pathogenic bacteria from thriving.
When powdery mildew was present, powdery mildew had a germination rate of 62%. With the application of a 3 day cattle manure compost tea extract, the germination rate reduced to 14% - a fantastic percentage change of -77%!
A more recent study from SARE echoed these findings, where the end result was a 85-93% decrease in standard fungicide applications.
Foliar Calcium Spray is recommended to help prevent blossom end rot and powdery mildew. A few studies back this up.
The Safer brand shares that a spray of 1 part milk to 2-3 parts water can help control powdery mildew in the squash and curcubit families.
Perhaps the most well-known cover crop that can reduce need for copper fungicide is Hairy Vetch.
This report from the USDA ARS shares how growing hairy vetch and then mulching with it before planting tomatoes reduces incidence of early blight.
A study published in Plant Disease Journal in 2002 extends the benefit of a hairy vetch cover crop followed by squash to prevent powdery mildew.
Hairy Vetch isn't the only cover crop that can help prevent foliar disease. Mustard greens and other brassicas have been shown to decrease blight incidence in squash.
Biodynamic Tree Paste is applied after leaf drop on orchard trees and grape vines. Andrew Faust talks about it in his interview on the Urban Farm Podcast.
He shares that the combination of clay and cow manure put into the tree paste is antimicrobial, antifungal, and antibacterial.
Supa Biotech in India notes that in their experience, the biodynamic tree paste provides more protection than the Copper Bordeaux Mixture typically applied in organic orchards. They use the tree paste to help control fungal gummosis, fungus growth, and root rot.
Purdue University notes that a 70% concentration of neem oil is known to kill powdery mildew spores.
In this lecture, the JADAM treatment for powdery mildew is outlined. Follow the safety precautions necessary when making JS. In this video he recommends safety goggles and protective gloves, but I would add on a mask as breathing in sulfur is not advisible.
Purdue University outlines these steps in order to prevent synthetic or organic fungicide applications:
At the end of the day, that's up to you.
Personally, at this point in time I am trying to create a healthy soil microbiome and pollinator population, and on top of that, I don't like spraying stuff that I need protection from.
BUT - for the average regenerative gardener or no-till homesteader, there are alternatives. Compost applications, compost tea, foliar calcium sprays, milk, hairy vetch, mustard cover crop, biodynamic tree paste, and neem oil all can help keep your plants, animals,and soil life healthy and happy.
Additionally, an ounce of prevention is worth a pound of cure. Be sure to rotate crops, offer sufficient air flow, trellis properly, surface water, and choose resistant varieties to help prevent issues from happening in the first place.
