Prepare to Get Ahead of Algal Spot

Josh McGill Diseases, Tip of the Week

By Megan Dewdney

Last summer, I received many calls about algal spot, especially in late June to early July.

Algal spot is a disease, caused by the algae Cephaleuros virescens, that occurs mostly on citrus tree limbs. Much of the year it is difficult to see because it is a gray-green color that blends into the color of citrus bark. However, in mid-summer it begins to bloom and becomes bright rusty red in a ring or donut pattern. This grabs the eye of the least observant individual!

Algal Spot
Algal spot lesions have a rusty red sporulation pattern of algae.

The lesions are usually about ½ inch but can connect to ring a branch. While seemingly benign at first, lesions can crack and crumble bark, stunt limbs and kill branches up to 2 inches in diameter. Spots can also be observed on leaves and over-ripe fruit, but these are much less damaging and can be easily removed.

Historically, algal spot was a minor concern in semi-abandoned groves, but it now occurs statewide in well-managed groves. So how do growers manage this re-emerging problem? For the best results, three applications of 0.5 gallons per acre of a phosphite salt with a fungicide label are recommended. 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 algae are fruiting and red (June and July).

In 2021, University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) researchers looked at the effect of spray timing with phosphite (ProPhyt, 0.5 gallons per acre). In this trial, it appeared that three applications of ProPhyt (3x) had equivalent activity on the fruiting of the algae 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.

Algal Spot
Bark cracking can occur from algal spot.

All ProPhyt treatments reduced the severity of symptoms, including bark cracking. More details on the trial will be published in the February 2022 issue of Citrus Industry magazine. While the results from two applications look as good as three in 2021, another year is needed to confirm that result. The good news is that the phosphite treatments have been the best in the 2019 and 2021 trials, which makes me confident that the recommendation for its use is sound. UF/IFAS researchers are continuing to look for additional effective compounds to ensure growers have rotational products.

Based on available information, the recommendation for algal spot management is the three-application program with 0.5 gallons per acre of a phosphite product with a pesticide label. 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 University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.

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