Fruit Size and Spraying Interval Are Key for Canker Control

The size of fruit that should be sprayed, the spraying interval and ways to avoid copper phytotoxicity were among the citrus canker topics plant pathologist Megan Dewdney offered growers recently.

Dewdney said fruit are most susceptible to canker when they are between 3/8 inch and 1.5 inches in diameter. The fruit rind becomes much more resistant when the fruit is more than 1.5 inches in diameter, she said. She added that rains in April, May and June promote early-season infection.

The University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) researcher recommended spraying with copper for canker control at 21-day intervals for several reasons:

• Copper spray is significantly reduced by rain washing.
• Copper doesn’t move once it has dried.
• Copper residue is cracked by fruit growth.

As the fruit grows, copper must be reapplied to continually cover the fruit as it becomes larger, Dewdney said. Spray volume and a slow tractor speed are important for fruit coverage.

Dewdney offered the following tips for proper application of copper:

• Use the label rate recommended for copper.
• Be cautious when the temperature is above 94 degrees because phytotoxicity can occur more easily in hot weather.
• The potential for phytotoxicity can be reduced with greater water volume per acre.
• Complex tank mixes, oil applications and nutritional materials contribute to phytotoxicity.
• The best application method is with an air blast sprayer; aerial applications provide inadequate penetration of canopy for control.

Canker suppression improves yield and helps keep fruit for harvest, Dewdney stated. She reported that wind-blown rain carries the canker inoculum to uninfected plants. She added that hedging, pruning or other activities that cause injury create wounds for infection and/or transmit the bacteria mechanically.

Dewdney works at the UF/IFAS Citrus Research and Education Center in Lake Alfred. She made her Feb. 24 presentation to numerous growers and others virtually, via Zoom. In the same presentation, she also discussed scouting and management for citrus black spot and postbloom fruit drop. Her presentation was hosted by Mongi Zekri, UF/IFAS citrus Extension agent.

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