Phosphite Best for Algal Spot

Ernie NeffDiseases

algal spot
Bark cracks from a limb affected by algal spot. (Photo by T. Weeks)

Algal spot had rarely been seen in Florida groves until fairly recently, Megan Dewdney reported in a virtual Florida Citrus Growers’ Institute presentation. The University of Florida Institute of Food and Agricultural Sciences plant pathologist said she started hearing from concerned orange growers within the past five years.

Those growers told her that copper, the historical recommendation for the disease, was not helping. So, she conducted what was likely the first trial on algal spot in decades. She learned that blackberries in the Southeast were having some success with phosphite, so she conducted trials with phosphite, copper and oxathiapiprolin.  

Dewdney got the best disease reduction from phosphite. The disease did not respond to copper.

The plant pathologist provided much background information for algal spot. She said most management information is from the late 1960s and early 1970s. Historically, the disease could usually be managed with improved routine grove maintenance.

Because the disease is associated with tree stress, Dewdney said that reduced plant stress through cultural practices is recommended. Optimal nutrition and irrigation, along with thinning of canopies for increased air movement, are the recommended cultural practices. “Unfortunately, we already have quite thin canopies and plenty of air movement,” she said. She added that keeping other diseases and pests under management “will help reduce tree stress and aid with this problem.”

Dewdney said early infections of algal spot are difficult to detect and are not easily visible to the naked eye. The disease occurs on branches, especially scaffold limbs. If conditions are favorable, scaffold limbs up to 2 inches can be killed.

“Wind is the main means of spread,” Dewdney reported. She added that rain and moving water, insects and agitation of the tree canopy can aid in algal spot spread.

Dewdney warned algal spot is likely to intensify with increased average temperatures, caused by climate change, over the next few years.

See Dewdney’s presentation here.

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About the Author

Ernie Neff

Senior Correspondent at Large