By Megan Dewdney
Every summer, a low rumble about algal spot reaches my ears. And every year, someone discovers it in their Florida grove for the first time. While this is a disease that is not always at the top of mind for growers, when you see it for the first time, it can be dramatic.
DISEASE SYMPTOMS
Algal spot is a disease caused by the algae Cephaleuros virescens occurring mostly on citrus tree limbs. It is difficult to see for most of the year because it is a gray-green color that resembles the color of citrus bark. In mid-summer, often starting in June, the color changes as the algae begins to bloom, becoming bright rusty red in a ring or donut pattern.
The lesions are usually about ½ inch but can often join to encircle a branch. The red donuts may seem harmless at first, but bark can crack and crumble under lesions, limbs stunt and branches up to 2 inches in diameter die. Spots can also be observed on leaves and over-ripe fruit, but these are much less damaging and can be easily removed.
NEW RECOMMENDATION
Originally, I recommended three applications of 0.5 gallon per acre of a phosphite salt with a fungicide label. The timings for the three applications are 1) dormant spray (late January and mid-February), 2) post-bloom (late April to mid-May) and 3) while the algae are fruiting and red (June and July).
However, in 2021, we looked at the effect of phosphite spray timing (ProPhyt 0.5 gallon per acre). In this trial, three applications of ProPhyt (3X) had the equivalent activity on the fruiting of the algae (blue bars in the graph) to a program of the two later applications (ProPhyt 2X). A single application (ProPhyt 1X) in late June substantially reduced the fruiting but not as much as the other two treatments.
All ProPhyt treatments reduced the severity of symptoms (orange bars in the graph), including bark cracking. Considering the cost of applications, it is difficult to recommend the dormant application without a substantial reduction in disease. Consistent trial results from the phosphite applications are promising but they do not eliminate the algae. It will be a multi-year effort.
With the information available, my latest recommendation for algal spot management is the April-May and June or July treatments program with 0.5 gallons per acre of a phosphite product with a pesticide label. Not all is lost if you notice a problem for the first time over the summer; one or two applications appears to reduce the bloom and hopefully further spread.
Megan Dewdney is an associate professor at the UF/IFAS Citrus Research and Education Center in Lake Alfred.
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