Copper Sprays for Plant Disease Prevention: Why, What, How

Winter application of copper sprays, while trees and shrubs are not actively growing, can prevent or reduce many significant diseases.

Why is applying copper spray important?

Copper sprays are very useful for dormant applications.  They provide suppression or control of both fungal and bacterial diseases.  Spray fruit trees and berries to reduce scab, mildew, leaf and cane spots, and bacterial blights.  Dormant sprays can also help subdue rose black spot, lilac bacterial blight, and dogwood anthracnose on ornamentals.

What form of copper is best to apply?

Not all copper fungicides are equal. All provide prevention (not treatment) of a variety of fungal, bacterial, and algal problems. Copper fungicides are available in a variety of formulations, and each has some advantages and disadvantages for certain seasons, crops, or diseases.  For example:

Basic Copper Sulfate Powder

Mix the powder with water, then spray onto fruit trees, trees, ornamentals, small fruits, and vegetables; or apply as a dust on ornamentals or vegetables for preventing diseases. It offers a fairly strong protection, but it is harder to mix in the sprayer, washes off a bit more easily, and is more likely to cause damage to plants during prolonged heat or moisture, or on tender new growth, so it is best used on dormant trees and shrubs, or on young seedling vegetables. It is suitable for only very limited applications through the growing season.

Copper Octanoate, also known as Copper Soap

This is a fairly weak form of copper in a thick liquid form, but tends not to wash off and is less likely to cause plant damage. It can be used on houseplants, ornamental plants, vegetables, herbs, and fruit and nut producing plants, and is suitable for both dormant applications and (with care) active growing season treatments.

Copper Diammonia Diacetate Complex

It is a very strong copper fungicide in a thin liquid form, and mixes easily. It is moderately resistant to washing off, but must be used with careful timing and with limits on total applications per year to reduce copper accumulations and plant injury. It is suitable for dormant applications only on deciduous fruit and nut producing plants, but can be used in limited amounts during the growing and production seasons on citrus, olives, and some vegetables. It is a particularly good bactericide, reducing ice nucleation in fireblight and lilac bacterial blight.

With copper products, as with other pesticides, always remember to read the label for specific instructions on concentration, timing, application, and precautions.

Timing of Spray Applications

For successful dormant sprays, keep in mind a few guidelines:

  • Plants should be dry when spray is applied. Even light sprinkles or heavy dew can over-dilute the spray, reducing effectiveness.  Wait until branches are no more than moist, not dripping.
  • Do not spray when the air temperature is below freezing, or is expected to drop below freezing before spray dries.  Freezing weather on wet spray can damage buds.
  • Spray will usually take 4 to 6 hours to dry.  During that period, even a light shower will wash it away, requiring a new application.  Even after the drying period, heavy rains impact spray effectiveness – re-apply a dormant spray if it gets washed away in the first 2 to 3 days after application.

For more information on spray schedules for fruit and nut producing trees, check out our informative handouts online – SPRAY SCHEDULE HANDOUTS.

Applying Copper Fungicide Video

This tutorial has the need-to-know information for you to successfully and safely apply copper fungicide to shrubs and trees. Get ready with a pad & paper, or bookmark this video for quick viewing as you prepare to apply fungicide to your fruit trees, blueberries or other fungal prone woody plants during their dormant season.

Fruit Tree Spraying Class Recording

Watch our class recording to learn from Darren Morgan proper techniques, timing, and products for fruit tree spraying. Proper care in the dormant season can alleviate disease and insect problems during the growing season

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