Less than 2.5 inches of rain occurred between the copper applications and the time trees reached Tight Cluster. Copper residues were not adequately weathered by Tight Cluster. Sprayers were not properly calibrated and application rates were too high. Spring frosts occurred just before or after copper applications. As plant cells freeze and thaw during spring frosts, copper ions may get pulled into cells and cause more damage than they would in the absence of frosts.
Copper helps control fire blight by reducing inoculum from overwintering cankers.
Apply copper sprays no later than Green Tip. Use the low end of the range of labeled rates, especially in years when spring is late and trees may move very rapidly from Green Tip to Bloom. Avoid applying copper just before predicted frosts or within several days after frosts have occurred. There is a small possibility that fire blight bacteria can be moved in symptomless nursery trees if the nursery trees are grown in regions where fire blight is prevalent in the surrounding orchards. To minimize the likelihood that streptomycin-resistant fire blight will be introduced into New York on nursery stock, we recommend that all newly planted apple and pear trees receive two applications of copper in the year they are planted and again the following year.