By Megan Dewdney
Algal spot is rarely a top concern for growers until it seems to be suddenly everywhere.
Is the disease a major concern or a nuisance?
It depends. If the disease is only in a few locations, then the management goal is making sure that the disease is not spreading too widely. If many trees are affected, the problem could be more serious.
What are the hazards of ignoring algal spot?
The major problem is the damage Cephaleuros virescens is doing to the bark and cambial layers of branches. It can girdle branches up to 2 inches in diameter, causing a significant loss of canopy. Even if the branches are not girdled, the bark can crack and, in severe cases, start to fall off in small chips, like a shattered windscreen.
In some groves, lesions on the stems of fruit have been observed, slowing the growth of those fruit. It is difficult to put a monetary value on these types of damage, but yield reduction is undoubtedly happening.
When is the disease observed?
The alga is most prominent in the late summer when it starts to sporulate. That is when it forms the bright red, ½ inch donuts on branches. These rings can join to form larger areas. The remainder of the year, the alga is a grey-green color that resembles other common harmless organisms seen on bark like lichens. Fruit and leaf symptoms are uncommon and not particularly damaging.
What should a grower do if algal spot has reached a level that requires management?
Generally, the recommendations are for two applications of 0.5 gallon per acre of a phosphite salt. These applications should be after bloom in late April to mid-May, followed by an application when the red fruiting bodies are visible in June or July.
These applications can also be part of a phytophthora program if that disease is also problematic. If the problem is only recognized once the red color has occurred, it is still recommended to start treatment. The control will not be as good as with two applications but will improve the outlook for the following season.
It is important to use a phosphite product with a fungicide label. The concentration is normally higher than for fertilizer products, so the apparent cost savings are usually lost by needing more.
While algal spot is a frequent nuisance, it is possible to keep it to low levels so that it does not eat away at yield. See more algal spot articles here.
Megan Dewdney is an associate professor at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.
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