Algal Spot an Increasing Problem in Florida Groves

Algal spot, not frequently seen in Florida citrus groves before 2015, has become more severe in recent years, University of Florida Institute of Food and Agricultural Sciences speakers reported Aug. 30. And the disease is likely to intensify as a problem with average temperatures increasing, said Megan Dewdney, associate professor of plant pathology at the Citrus Research and Education Center in Lake Alfred.

SYMPTOMS

Multi-county citrus Extension agent Mongi Zekri, who works in Southwest Florida, led off a presentation by Dewdney with an overview of algal spot symptoms. He said the disease, mostly found in sweet oranges, is on the branches and most visible when the causal green alga is producing fruiting bodies from June to September.

An initial symptom is branch thickening around the algal spot lesion. If conditions are favorable, the disease can kill branches of 2 inches in diameter or stunt branch growth, causing chlorotic leaves and leaf drop.

Zekri said symptoms on fruit should not present a problem because they usually develop late on overripe fruit. Lesions are dark black and circular to irregularly shaped.

Dewdney added that early algal spot infection is difficult to detect and is not visible to the naked eye. The disease has been a minor problem for most cultivars, except lemons and limes, she noted.

CONTROL

Algal spot is associated with tree stress and is more frequent and severe on stressed or declining trees, Dewdney reported. She recommended reducing tree stress through cultural practices, including optimizing nutrition and irrigation, and through disease and pest management for other problems.

Trials that Dewdney and associates have conducted in recent years showed that phosphite products offered the most consistent control for algal spot over three years. Dewdney recommended three applications of phosphite products if the problem is severe in the first year. In following years, two applications will be sufficient. She said if a problem is detected in late summer, one application will help for the next year. 

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