Early season copper sprays-to spray or not to spray?

One of the most frequent questions that apple growers ask at this time of year is whether or not they should apply an early season copper spray. Fire blight can be a devastating bacterial disease of apples and pears. Currently there is very little published research supporting the effectiveness of copper in supressing Erwinia amylovora, the bacterium that causes fire blight. Copper sprays applied at the green tip bud stage of apples for scab control may have the potential to reduce the spread of fire blight inoculum from overwintering cankers.

Growers that have had fire blight in their orchards over the past two years may want to consider using early season copper sprays to reduce the spread and colonization potential of bacteria coming from cankers overwintering in the orchard. Orchards that have not had fire blight infections over the past few years usually do not have overwintering cankers, and thus would not benefit from the use of copper sprays except for scab control and possibly other types of wood rotting infections like anthracnose (fiddlestring canker). The application of copper does not kill the bacteria. The copper residue provides an unfriendly environment over the bark and bud surfaces of the trees, preventing bacteria from getting established. The bacteria that cause fire blight are dispersed through the orchard before bloom when the initial infections occur. Generally, the greatest flux of bacteria onto the bark occurs at the tight cluster to the pink stage of apple.

Unfortunately in years with heavy rainfall between green tip and half-inch green to tight cluster, the copper residue may be partially washed off, thereby diminishing the effect of the treatment. David Rosenberger at Cornell University cautions the use of copper applications after the half-inch green stage since they can result in phytoxicity and severe fruit russetting depending on the formulation.

In order for copper sprays to work effectively it is recommended they be applied to all of the trees in the block (not just susceptible varieties). Untreated trees provide a safe haven for the bacteria. Insect vectors can move the bacteria around easily. Copper sprays should be applied in high water volumes to achieve a washing action on bark surfaces (at least 1000 L/ha).

Some copper formulations have been shown to be less likely to cause phytoxicity. These formulations are grouped together as fixed coppers. Formulations of copper that are finely ground like the fixed coppers (copper oxychloride, copper hydroxide) tend to have better retention on plant surfaces and may have longer residuals resulting in better protection from bacteria.

For more information on the use of copper on fruit trees refer to OMAFRA publication 360: Fruit production recommendations.

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