By Steve McGrane MAIH RH Images/ Steve McGrane
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.
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.
Copper is a naturally occurring element termed a heavy metal, and generally occurs at very low levels in the natural eco system but accumulates in the environment largely through horticultural and commercial activities.
Note: A heavy metal is best described as an element that is dense in volume and resistant to break down. While copper can bind with proteins and remain biologically bound, it can become ‘free’ (as occurs in the use of copper fungicide preparations) and becomes toxic. Copper accumulates in soils, seabeds, humans and plant tissue.
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.
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.
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.
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.
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.
Liquid Copper. Image/ Steve McGrane
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.
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.
Effective antifungal treatments with minimal environmental effects are as follows (Alva AK, Huang B, Paramasivam S, 2000):
Combine a ‘potassium based’ fungicide with plant oils for stronger efficacy. Image/ Steve McGrane
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).