By Erica Faber – Orchard Productivity Manager
Excessive copper can affect the uptake of phosphorus, zinc, manganese and iron, resulting in low levels within the plant contributing to poorer performance and yield. High levels of copper have also been found to inhibit root growth as well as have an adverse effect on beneficial fungi (which suppress phytophthora), bacteria, protozoa and earthworms. Copper-based fungicides however are an effective means of improving packouts and returns by controlling fungal diseases such as anthracnose, post-harvest rots etc. Until alternative, effective controls are found, it seems they are our only option. So, what can we as growers do to ensure we try minimize these negative side effects.
These are some suggestions for avoiding elevated levels of copper and their side effects within our orchards:
Firstly, by managing orchard sanitation, we can reduce the source of fungal inoculum in orchards by chipping or mulching the old prunings and by removing windfall fruit from the orchard floor. This will lower the incidence of anthracnose and post-harvest rots and will reduce the amount of copper sprays eventually required for the control of these fungal diseases.
Pruning also plays an integral part in reducing the severity of these fungal diseases by allowing better air movement throughout the canopy, thereby improving drying conditions.
Calcium sprays onto young developing fruit will be absorbed by the fruit and ensure improved fruit quality as well as strengthened cell walls. These strengthened cell walls are more resilient to penetration of the fungal hyphae as well as post-harvest bruising. Remember though that Boron also affects calcium absorption, so this needs to be in balance for the calcium to be effective.
Limit the copper ion concentration on plant surfaces by using copper products that are relatively insoluble in water, i.e. fixed copper.
- Copper Oxides (e.g. Nordox 75 WG™, AgCopp 75)
- Copper Oxychlorides
- Copper Hydroxides (e.g. Kocide® Opti, Champ® DP, Hortcare® Copper Hydroxide 300)
- Copper Sulphates (e.g. Tri-base Blue)
- TEPA Chelated Copper (e.g. Coptyzin)
Soluble coppers erode faster and require more frequent applications as they are less persistent. Less soluble coppers are more persistent and release a supply of copper ions for a longer period of time. These less soluble formulations require less frequent re-application thereby improving efficacy and reducing excess build up in soils.
Copper can accumulate to high levels on plant tissue when sprayed repeatedly to cover new growth during periods of no rain. In this situation, after a rain event, a large amount of copper ions may be released leading to phytotoxicity or contamination of the soil.
Timing copper sprays to conditions of spore release and conducive weather conditions will ensure copper is only sprayed when it will be effective and thereby reduce sprays that are ineffective and only add to unnecessary copper excesses.
Application of humates also “stabilizes” excess copper so that it is less reactive and/or hazardous to both the soil microbiology and the plant.
If your current copper levels are high resulting in for example, limited uptake of phosphorus or zinc, regardless of their soil levels, foliar applications of these nutrients can be applied. This will help to correct these imbalances or deficiencies in the plant until the soils are more balanced.
All the above can assist in reducing the high copper levels in the soils and with it the resulting problems associated with nutrient imbalances and soil health and fertility. An integrated approach to managing fungal diseases will not only ensure improved returns by reducing the incidence of post-harvest rots but also a more sustainable approach to growing avocados.