Copper sulphate is a fungicide used to prevent and control plant fungal diseases, including powdery mildew, leaf spots and blight. When mixed with lime and water as a preventive treatment, it is known as Bordeaux mixture. Copper sulphate is available in fluid concentrates, as a soluble powder and a dust.
Fruit trees are susceptible to disease during bloom and when rain or sprinkler watering wets the tree's foliage, spreading disease and creating favourable conditions for infection. Spray fruit trees shortly after the dormant season begins in the autumn, after the leaves have fallen but before winter rains begin. Pests and diseases are exposed and more vulnerable during the dormant season. A second application of copper sulphate is recommended at bud swell, just before bloom.
Copper sulphate application rates vary by product and by fruit type. Too much copper can be poisonous to some plants. Follow product guidelines for mixing rates and application procedures. Copper sulphate will remain on leaves for one to two weeks, or until washed off by heavy rain or irrigation. Spray when your forecast calls for several days of dry weather.
Follow all precautionary statements on product labels. Wear goggles, a mask, protective clothing and gloves when handling copper sulphate products. At normal application rates, copper sulphate can be poisonous to chickens and sheep. Earthworms are also sensitive to copper, as it may accumulate in their tissues. Copper sulphate is one of several copper-based treatments for fruit tree pests and diseases. Know which pest or disease is affecting your trees to ensure you select the right product for your problem.
To limit the use of chemical insecticides and fungicides, plant disease-resistant fruit tree varieties; prune fruit trees to allow sunlight to penetrate the limbs and to dry leaves quickly; avoid overhead irrigation; remove damaged shoots, fruit and leaves; and space trees to allow for adequate air circulation. Keep fruit trees well-watered but not overwatered, and fertilise them regularly. Use mulch to keep soil moist.
Sources differ on whether products derived from copper, including copper sulphate, should be used with an organic gardening program. Some prefer to exclude copper-derived products from the organic garden and instead use less toxic products to prevent and control fungal diseases. However, some experts count copper-derived fungicides and bactericides as appropriate for organic use. The Henry Doubleday Research Association's "Encyclopedia of Organic Gardening" also includes Bordeaux mixture as one of its permitted pesticides.