However, there is also a risk that plants treated with these salts will accumulate the copper and that it will eventually reach toxic levels.
In the experiments reported below, cucumber plants were grown in a glasshouse on rockwool slabs irrigated with a copper-free nutrient solution. Copper sulphate was then added to produce concentrations ranging from 0.55 to 2.05 milligrams of copper per litre.
No foliar symptoms of copper toxicity or deficiency appeared for at least six weeks, but after 10 weeks, the leaves of plants growing in the highest concentration of copper were less green.
Fewer fruits were picked from this treatment and their average weight was lower. The plants growing in any solution containing more than 1.05 milligrams of copper per litre had brown roots, which suggested that root injury might be a more sensitive indicator of copper toxicity than leaf colour.
Therefore, if copper salts are used as a fungicide on cucumber stocks, the plant roots should be examined regularly for brownness because this could indicate that the plant's copper concentration is approaching toxic levels.
Greenhouse Cucumber Growth and Yield Response to Copper Application by Zheng, Wang, Cayanan and Dixon (2010). HortScience 45 (5): 771-774. ISHS members can view HortScience from the website www.ishs.org