What To Do About Algal Spot

Tacy CalliesDiseases, Tip of the Week

algal spot
Algal spot lesions

By Megan Dewdney

I did not need to visit my field trial to know that algal spot was in bloom on citrus in the last month. I have received many phone calls about it recently as growers notice the bright orange doughnuts on their tree limbs.

For those unfamiliar with algal spot, it is a disease that occurs mostly on citrus tree limbs. It is difficult to see during much of the year because it is a gray-green color very similar to the color of citrus bark. However, in mid-summer it begins to bloom and becomes bright rusty red in a ring pattern. These lesions tend to get growers attention! The lesions are usually about ½ inch but can join 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 removed.

Bark cracking from algal spot

Historically, algal spot, caused by the algae Cephaleuros virescens, was a minor concern in semi-abandoned groves. But recently we have seen it pop up statewide in well-managed groves. The old wisdom on algal spot management was that once a good copper program was in place, the disease would no longer be a problem. Through my own experience, but also from grower reports, copper is no longer effective against this organism. The reasons behind this are uncertain.

Coincidentally, as this disease emerges in citrus, it has also emerged in blackberries in the southeastern United States as a problem that damages the fruiting canes. I was able to use the information developed on blackberry to select some chemistries for the first citrus trial in decades. The field trial was conducted in 2019 on Valencia trees with moderate to severe symptoms.

algal spot

We put out three applications of each product at three timings: 1) Dormant spray (Feb. 5), 2) post-bloom (May 9) and 3) when the algae were fruiting (red color, June 20). I tried copper hydroxide (Kocide 3000), a phosphite (ProPhyt) and an anti-omycete product (Orondis). A half or quarter gallon rate of ProPhyt (see graph above) showed the greatest effect. From this limited data, I am recommending growers use 0.5 gallons per acre of a phosphite (54.5% active ingredient) labeled for disease management. I am not sure if an application at this time of year will have a substantial effect on algal spot, but it would not be harmful and is an option.

Hopefully, my trial this year will give me some insights on application timing. Currently, if you observe significant algal spot in your grove, I recommend planning for a management program to start in late January for the three applications described above. As this is an emerging disease, I need information from growers about their experiences. Please fill out this questionnaire about algal spot so I can better tailor trials to your needs.

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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