Copper-based fungicides
These have developed and changed over the years; here are some pointers to help use them effectively.
• Copper-based sprays are protectant fungicides. They form a protective layer on the leaf and fruit surface that inhibits disease spore germination; hence they need to be applied prior to damage symptoms occurring.
• Copper-based sprays are not systemic – they are not absorbed into the plant. They only protect the part of the plant where the spray lands and sticks. So, it’s important to get thorough coverage over all the foliage and fruit.
• As leaves and fruit grow, the new tissue is not protected, and wind and rain erode the copper coverage over time. So the fungicide may need to be applied again, depending on the severity of disease, weather conditions and the copper formulation used.
• Nowadays, a variety of copper formulations are available – including wettable powders, wate-rdispersible granules, liquid-flowable suspensions and aqueous liquids. as a rough guide, the particle size of the active copper ingredient in each goes from wettable powders having the largest particles to aqueous liquids having the smallest.
• Particle size counts. Research has found that the smaller the particle size of the formulation used, the more effect it is at controlling disease. This is the most important quality for a formulation, along with how well it sticks to the plant surface and its rain-fastness (resistance to rain).
• Some research also suggests that frequent applications at lower dilution rates may be more effective than fewer applications at a higher dilution rate.
• Copper sprays can damage citrus foliage if applied at excessively high rates, so make sure to follow the product label recommendations. copper spray damage typically shows as black speckles on leaves and fruit skin.
• In my experience, aqueous formulations, like Yates Liquid copper, are much easier to measure accurately and mix than the copper oxychloride wettable powder of the ‘good old days’. always read and follow the product label carefully. Some copper formulations may be mixed with insecticide oil or liquid fertilisers, while others must not be.
• Copper sprays have no withholding period; just wash the fruit before eating.