By Gordon Johnson, Extension Vegetable & Fruit Specialist; firstname.lastname@example.org
There has been considerable use of copper fungicides this year, particularly because of the wet weather. Fixed coppers (low solubility) are an important component for bacterial disease control and also are effective on a number of fungal and oomycete diseases.
In the past, phytotoxicity to crops has been an issue with copper applications. Newer copper products have proven to be safer on vegetables and fruits. However, toxicity can still be a problem in some situations.
Copper fungicides work to kill pathogen cells by denaturing enzymes and other critical proteins. However, copper can also kill plant cells if absorbed in sufficient quantities. Low solubility “fixed” copper fungicides, when sprayed on plant leaves will dry and then will release copper ions in small amounts every time the leaf is wetted. These released copper ions are what will kill pathogen cells and also have the ability to kill plant cells. To cause damage to plant cells, the copper ions must move from the plant surface and penetrate plant cells.
Leaves have a protective waxy layer, the cuticle, which prevents entry of copper ions. Copper ions can enter plant cells where cuticles are thin (as with new leaves), through stomates, and through hydathodes on leaf margins. We often see copper injury on new leaves and leaf edges because of this.
Because copper ions are released when copper residues are wetted, we also see more injury when copper is applied to wet leaf surfaces or in slow drying conditions.
Because copper ions can also enter through stomates, anything that would increase the spread of copper over the leaf and increase the entrance of copper ions through stomates and cuticles will increase injury. Research has shown that adding surfactants with copper fungicides greatly increased the injury to foliage. This is because the surfactant allows the copper to spread over more of the leaf surface and contact more stomates as well as to penetrate more through cuticles.
Because of this, copper products should only be used with great caution with surfactants. This includes some spreader-stickers and other fungicides which contain their own surfactants or spreaders. This is the reason that with fungicides such as Ranman, where a surfactant is recommended for Phytophthora control, we caution to not apply that fungicide with copper.
Copper also becomes more soluble in acidic conditions and should not be used with spray acidifiers or other acid forming products.