What are they? Are they safe for the environment? How to use copper effectively in a horticultural setting.. the alternatives.

Fungal infection on a geranium. Image/ Steve McGrane

Fungal infections can be highly impactful to many plants, including vegetables, fruit and ornamentals. Copper is an effective fungicide for controlling fungal, viral and bacterial infections. However a large body of research indicates copper fungicides have a considerable impact to humans and the environment.

The reality is, many treatments used in horticulture have detrimental effects. We can’t entirely stop the use of such chemicals so being ‘well informed’ and understanding effective alternatives, is the most practical course of action.

How Does Copper Work as a Fungicide?

Copper impairs cellular functions in viruses, fungi and bacteria through ‘peroxidation’ (oxidation) of cells leading to inaction by lipids resulting in the collapse of cells walls, attacking virus structural integrity, inhibiting protein and enzyme chemical disruption by binding to surfaces. These actions either prevent fungal spores from completing essential metabolic functions or cause desiccation (drying out) through destruction of cell walls.

Implications of using Copper in Horticulture

Copper is a Heavy Metal

Low Levels of Copper Kills Soil Biota

At very low levels copper becomes toxic to plants, humans and importantly, soil organisms, including worms. Relatively low concentrations prove toxic to soil organisms (Helling et al. 2000).

Copper sprays are harmful to all aquatic life (including fish), livestock and soil biota. Small elevations of copper in the soil reduce beneficial mycorrhizae associations (Liao et al. 2003). Fungi are essential for symbiotic relations with plants.

Copper sulfate kills soil biota and, insects (e.g. bees) when sprayed as an anti-fungal (Bogomolov et al. 1996, Böckl et al. 1998). Impact on pollinators highlights the need to be aware of timing of applications. Spraying fungicides when pollinators are present is counterproductive and environmentally irresponsible.

A fungal disease that attacks kangaroo paws (Anigozanthos) is Alternaria alternata. This disease can be controlled in perennial varieties of kangaroo paws (having a rhizome), by cutting back leaves to within a cm of the soil, post flowering. Copper is also effective in addressing a single attack of the disease. However subsequent infections are better controlled through seasonal removal of infected leaves. Image/ Steve McGrane

Copper Anti-Fungal Compounds Encourage ‘Pest Resistance’

Research shows disease and pathogenic fungi develop resistance to copper treatments making copper fungicides ineffectual overtime and we should therefore have a mix of chemical and cultural strategies e.g. rotate crops or improve air flow in the garden or nursery. Refrain from overhead watering.

Copper Stays in the Soil

In most soils, copper residues are likely to remain indefinitely, and will continue to influence the health of the soil. This has implications for future land management and human health.

Organic Matter Does Not Lower Copper Levels in Soils

Thrupp (1991) found that copper, arising through Bordeaux application, tended to be associated with areas of high organic matter build-up. Hence even good levels of organic matter, will not assist in reducing copper toxicity but rather exacerbate. Hence the message is, use copper sparingly because you can’t put the genie back in the bottle.

Using Fungicides Effectively

Apply fungicides before the development of buds, or after in the case of liquid copper and ‘certified organic’ fungicides that are potassium based or other compounds such as milk that do not have a corrosive effect on plant tissue.

Applying fungicides before bud burst is also necessary to prevent fungal spores’ transferring to the plants androecium (male sexual parts of the plant) after buds open and develop further during fruit set and fruit development. Liquid copper (discussed further below) can be applied after bud burst without damaging tissue.

Examples of Common Copper Fungicides and Their Uses

A copper formulation used pre-emptively for the treatment of fungus (mildew and botrytis) is Bordeaux. Bordeaux uses copper ions and a slurry of lime to prevent fungal spores from germinating by blocking an essential enzyme.

Bordeaux is typically applied to deciduous trees in winter. Note: alternatively, ‘organically certified’ Bio Dynamic Pastes, are an effective alternative to Bordeaux when applied in winter.

Another common copper fungicide, Copper Oxychloride, controls fungal and bacterial attacks on trees and vegetables; This includes diseases like mildew, anthracnose, banana leaf speckle, leaf spot, septoria leaf spot, black spot, melnose, lemon scab, smoky (sooty) blotch, brown rot, collar rot and pink disease etc. Since copper oxychloride becomes toxic in the soil, use removable mats under plants when spraying, to avoid contamination of soil.

More recently ‘liquid’ forms of copper (copper hydroxide), have become popular as anti-fungal treatments. Liquid copper is more effective due to its inter-laminar nature (nano in size, entering leaf tissue) and therefore more persistent and less effected by wet weather. However it is also more persistent in soils.

Note: fungicides are only disruptive but not a cure, as fungal spores survive treatments.

Copper, can be too Much of a Good Thing, even for plants!

Although copper is required in the soil for plant health, it easily becomes toxic causing iron chlorosis (yellowing) or stopping photosynthesis turning leaves dark green before becoming white. Excessive copper, damages roots causing wilt or death.

Note: ‘Lesser quality’ copper supplements, or anti-fungal products, may contain high levels of undesirable heavy metals such as arsenic, mercury, lead and cadmium.

Alternate Fungicides

Effective antifungal treatments with minimal environmental effects are as follows (Alva AK, Huang B, Paramasivam S, 2000):

• Use ‘covers crops’, to reduce pathogenic fungi as an alternative to copper.
• Ensure good air flow around plants and prune fruit trees in winter.
• Apply a Bio Dynamic Tree paste after pruning each year (4 parts cow manure, 2 parts diatomaceous earth or silica, 3 part potters clay, or garden clay, add rain water to make a slurry which is applied with a brush)
• Use a potassium based fungicide pre and post ‘bud burst’.
• In the nursery or garden, use drip irrigation instead of overhead watering to reduce fungal growth.
• Add regular amounts of compost and inoculated compost (beneficial fungi strains that attach to pathogens)
• Surfactants and bio surfactants (oils or fats used to lower surface tension and discourage leaf fungi) e.g. Eco Oil
• Compost teas (good bacteria prevent harmful fungal spores establishing)
• pH modifiers and bicarbonates (changing the pH prevents fungal spore germination) calcium and silicone sprays e.g. Milk products, create an alkaline surface not conducive to fungal spore germinations.
• Molasses; mixed at a rate of 1:10 in water and sprayed on plant leaves.
• Essential oils:
  – Citronella oil
  – Jojoba oil
  – Nimbin (neem oil)
  – Oregano oil
  – Rosemary oil
• Wettable Sulphur

Acknowledgements;

October 26, 2017, Society of Environmental Toxicology and Chemistry
Van Zwieten M, Stovold G, Van Zwieten L (2004b) Literature Review and Inventory of Alternatives to
Copper for Disease Control in the Australian Organic Industry. A report for the Rural Industries
Research and Development Corporation. RIRDC Project DAN-208A. ISBN 0 7347 1590 0, 101p
NSW Department of Primary Industries, June 2017
Kay T. Ho et al, Effects of micronized and nano-copper azole on marine benthic communities, Environmental Toxicology and Chemistry 2017.
A report for the Rural Industries Research and Development Corporation. RIRDC Project DAN-208A. ISBN 0 7347 1590 0, 101p. Ware GW (1978) ‘The Pesticide Book.’ (W.H. Freeman and Company: San Francisco).