Updates on Florida Citrus Diseases

Researcher Megan Dewdney recently provided updates on citrus canker, postbloom fruit drop (PFD) and algal spot. Dewdney is a University of Florida Institute of Food and Agricultural Sciences associate professor of plant pathology at the Citrus Research and Education Center.

CANKER
Fruit is most susceptible to canker when it is from 3/8 to 1.5 inches in diameter, Dewdney said. The fruit’s rind becomes much more resistant once the fruit exceeds 1.5 inches in diameter but is still somewhat susceptible throughout the growth period. Rain in April, May and June promote early-season infections.

Dewdney recommended starting copper sprays when the fruit reaches a 3/8-inch diameter. Applications should be made at no more than 21-day intervals to provide protection. The 21-day recommendation is made for several reasons:

• Copper residue is reduced by rain washing it off.
• Copper doesn’t move once dried.
• Copper residue becomes cracked by fruit growth. Copper must be reapplied to cover the fruit as it becomes larger.  

Dewdney said canker suppression improves yield by 10 to 15 pounds per tree, and canker control helps keep fruit for harvest.

POSTBLOOM FRUIT DROP
PFD was not frequently seen in Florida groves until recent years but is likely to intensify as a problem with increased average temperatures, Dewdney said.

The PFD fungus moves with rain splash and windborne rain; it prefers a humid climate. Equipment and workers can also move this disease. PFD is most severe in multiple blooms.

Dewdney recommended using the Citrus Copper Application Scheduler to time fungicide applications. She said treatment won’t likely be needed in most Florida locations this season since rain is predicted to be below average during winter 2022–23.

Removal of declining trees can help reduce PFD inoculum, Dewdney noted.

ALGAL SPOT
Algal spot is more frequent and severe on stressed or declining trees, Dewdney reported. She recommended reducing plant stress through cultural practices including optimal nutrition and irrigation. Use disease and pest protection management for other problems to help reduce tree stress, she suggested.

Based on trial results, phosphite products give the most consistent algal spot control over three years. Dewdney recommended three applications if the problem is severe in the first year; two applications will be sufficient in following years. If a problem is detected late in the summer, one application will help for the next year.

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