UF/IFAS Associate Professor of Plant Pathology warns Florida citrus growers to keep an eye out for a re-emerging disease: algal spot.

Algal spot is a citrus disease caused by a green alga, Cephaleuros virescens. It was once prevalent in Florida citrus groves, but UF/IFAS Associate Professor of Plant Pathology Megan Dewdney maintains that she has fielded more and more questions and concerns about the disease, according to a Citrus Industry article. Explore the disease, and what to look for, below.

Details on Algal Spot

Agal spot affects the branches of citrus trees. According to Dewdney, it’s not considered parasitic, but it causes damage to the tree if it goes untreated. The highlights of Dewdney’s article about algal spot include:

  • Sweet oranges are the most frequently affected
  • The disease is most noticeable on the branches of citrus trees when the alga is “producing fruiting bodies,” which is around June to September.
  • The disease creates lesions in the bark, which eventually cracks and falls off.
  • The “fruiting” alga colonies appear “orange red to dark red with a velvety texture. The lesions often have a donut appearance with a gray center surrounded by red.”
  • It affects fruit by giving it an overripe appearance with black circular or irregular lesions which make the fruit unmarketable.
  • Similar black, circular lesions are also seen on leaves.
  • Routine use of copper used to manage the disease, but that no longer seems to be effective for unknown reasons, according to Dewdney.
  • A trial of new treatments conducted by UF/IFAS in 2019 shows promise for “phosphite, ProPhyt, and the combination of ProPhyt and Kocide 3000.” According to Dewdney, but more data is needed for formal recommendations

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