Copper Based Fungicides
Copper based fungicides, such as copper hydroxide (or cupric hydroxide) and copper oxychloride, are commonly used in gardens and on farms. Some are certified organic inputs so can be particularly valuable to organic gardeners and farmers.
What diseases can be prevented or treated by copper based fungicides?
Copper based fungicides, despite being called fungicides, can treat and prevent a wide range of fungal and bacterial diseases including:
- black spot or apple scab on apples and pears
- shothole and freckle on apricots
- shothole on cherries and plums
- black spot, smoky blotch, melanose and lemon scab on citrus trees
- leaf curl and shothole on nectarines and peaches
- downy mildew on grapevines
- common blight on beans
- rust and chocolate spot on broad beans
- black rot, peppery leaf spot, ring spot and downy mildew on brassicas (such as broccoli, cauliflower, cabbages and may Asian greens)
- bacterial spot and bacterial canker on capsicums
- angular leaf spot and bacterial leaf spot on cucurbits (such as cucumbers, melons, pumpkins and other squashes)
- downy mildew, bacterial leaf spot and anthracnose on lettuces
- downy mildew on onions
- leaf spot on parsnips
- ascochyta blight and bacterial blight on peas
- target spot, early blight and late blight on potatoes
- downy mildew on silverbeet and spinach
- bacterial spot, bacterial speck, bacterial canker, target spot, septoria leaf spot and late blight on tomatoes
- bacterial leaf spot, black spot and sooty mould on ornamentals (such as roses)
How do I use copper based fungicides?
Each manufacturer makes their fungicide slightly differently so always follow the directions on the packaging. Generally, copper based fungicides are powders that can be dissolved in water or liquids that need to be diluted with water. Usually they have a short lifespan once they've been made up so only mix a batch when you're ready to use it.
If spraying a tree or a larger number of plants, it is useful to use a pump sprayer that will prevent your hands from getting tired. It's surprising how tired and sore hands can get if you use a trigger spray bottle! You might also consider using a pump sprayer with a 360° nozzle to help ensure you spray the solution on all plant surfaces. The disadvantage of this is that you might spray beneficial insects without seeing them; something should be avoided if you want to preserve your beneficial insect populations.
Spray on a mild, dry, still day. If you spray right before it rains, the fungicide will wash off and won't be effective. If you spray on a windy day, spray will blow all over you (not at all desirable) and will drift onto other plants that could be sensitive to copper and onto neighbouring soil. As high levels of copper can kill worms and other beneficial soil microorganisms, you want to use the minimal amount of fungicide and restrict it to the smallest possible area. Also, if you spray during hot weather or frosty conditions, plants may be damaged.
Always wash your hands well after spraying copper based fungicides. You might also consider washing your clothes afterwards as well or using overalls or other protective clothing.
When should I apply copper based fungicides?
Different plants should be sprayed at different stages of growth (such as bud swell (when flower buds are growing larger in preparation for opening but before they actually open), bud burst (when the buds open), at leaf fall or when conditions favour disease development (generally wet and humid weather with little wind (because it reduces air circulation)) so be sure to read the instructions carefully.
Are copper based fungicides safe?
In terms of the safety of eating food sprayed with copper based fungicides, all copper based fungicides have a withholding period of at least one day. This means that you cannot eat any produce harvested from a crop sprayed with the pesticide for at least one day after you have sprayed. After this time, it is considered safe to eat any such food. In terms of how safe it is to use copper based fungicides, all copper based fungicides can irritate human skin and eyes (and almost certainly the skin and eyes of any pets you might have). Breathing in the fumes can also cause nausea, vomiting, muscle aches, a dry throat and headaches. Repeated exposure over a long period can cause sensitisation (which means that you will have an allergic reaction, usually allergic contact dermatitis, to the fungicide every time you come into contact with it after you have become sensitised). Ingestion of copper based fungicides can cause irritation of the gastrointestinal tract, nausea, vomiting, salivation, gastric pain, hemorrhagic gastritis, diarrhoea, capillary damage, liver and kidney damage, central nervous system stimulation followed by depression, jaundice, pain in the liver, and haemolytic anaemia. As a result, always wear a long sleeved top, trousers and gloves when using copper based fungicides and wash any exposed skin as well as your hands thoroughly after use. You may also wish to wear a dust mask. You should also spray only on still days to prevent the pesticide from being blown into you. Perhaps most importantly, always keep copper based fungicides out of reach of children to ensure they cannot accidentally swallow some.
Copper based fungicides are known to be toxic to some species of fish so always take steps to prevent the fungicide from getting into local waterways. Steps can include not flushing excess solution down drains or into gutters. High levels of copper in the soil are toxic to worms so only spray copper based fungicides directly on plants and only when required.