Thoughts from On Top The Hill: Untangling the Web of Copper Fungicides

Fire blight season is approaching, and delayed dormant applications of copper fungicides are recommended for management of fire blight and other tree fruit diseases.

Below is a summary of copper fungicide formulations, their characteristics, and their limitations:

· Fixed Copper

o Fixed copper is safer for plant tissue than “bluestone” copper (see below).

o May be used throughout the growing season, but may cause fruit russetting.

o Low solubility in water, resulting in a lower risk for phytotoxicity. High rates of fixed copper may cause plant damage, however.

o Release copper ions slowly (with water/rain), resulting in longer residual activity. This extended release may damage plant tissue if prolonged.

o Slow drying time (e.g. rainy conditions) increases solubility of copper, release of copper ions, and thereby phytotoxicity of copper fungicides.

o Acidic conditions/additives also increase copper solubility, ion release, and phytotoxicity.

§ Adjuvants, phosphorus acid fungicides, and mancozeb lower pH of tank mixes.

§ Growers may add lime to reduce potential for plant damage.

o Use higher rates during dormancy for fire blight management (until ¼” to ½” green).

o Lower rates should be used during growing season for management of other diseases. Follow label instructions.

o Common forms of fixed copper fungicides:

§ Basic copper sulfate – Cuprofix, Basicop

§ Copper Hydroxide – Kocide, Champ

§ Copper oxychloride sulfate – C-O-C-S

§ Cuprous oxide – Nordox

· Copper sulfate pentahydrate – bluestone

o Dormant spray, only.

o Highly soluble copper ions can be phytotoxic to exposed plant tissue.

o Often combined with lime to help “tie up” copper ions and slow their release.

o No residual activity. Copper ions are released rapidly upon application.

o Common brands of copper sulfate:

§ Mastercop, Phyton

Copper fungicides are not systemic and wash off with rain. Fixed coppers have more residual activity than bluestone copper because they release ions slowly. Rapid release of ions can cause phytotoxicity, so growers are encouraged to avoid acidic solutions and slow-drying situations.

Copper is an antimicrobial; it is non-selective. Exposed bacteria, fungi, and even plant tissue are susceptible to damage. Thus, high rates of copper are not recommended after green-up. Additionally, copper can accumulate in soils, where it can inhibit plant growth and adversely affect microorganisms and earthworms.

Metallic copper equivalent is the amount of copper available for fungicidal activity (different from active ingredient). Basic copper sulfate usually contains the highest metallic copper equivalent. Refer to label for more information.