Florida’s citrus growers must always be on the lookout for new pest concerns, and algal spot is high up on that list. It started showing up in citrus groves in 2015, and it has grown more serious in the last few years. UF/IFAS researchers have been prepping for algal spot through multi-year trials, and they have found that tree nutrition can play a big role in mitigating the disease.
Spotting Algal Spot and Impacts
Algal spot is mostly found in sweet oranges, and also lemon and limes, and it is most visible on citrus tree branches from June to September when the green alga that causes the disease is producing fruiting bodies.
Other early symptoms include a thickening of the tree branch around the lesion caused by the algal spot. If the disease is allowed to progress, growers could see chlorotic leaves, leaf drop, stunted branch growth, and the death of branches up to 2 inches in diameter.
Algal spot can also cause lesions on fruit that are dark and black and circular to irregular shapes. However, it’s usually not a huge issue as these dark lesions don’t usually develop until the fruit is overripe and not marketable.
With all that said, the early stages of the infection are not noticeable with the naked eye, so UF/IFAS researchers recommend growers be as proactive as possible.
Nutrition and Treatment Options
UF/IFAS researchers maintain that algal spot is linked to tree stress; trees that are stressed or declining are much more likely to have algal spot and it is more likely to be severe. This is where nutrition comes in.
Preventative measures for algal spot include using cultural practices to reduce tree stress. Utilizing optimal nutrition is an important part of combating algal spot. Other options for reducing tree stress include optimal irrigation, managing other diseases and pests, weed management, and more.
In terms of treating algal spot, trials conducted by UF/IFAS revealed that phosphite products best controlled algal spot. Recommendations include three applications of phosphite products in a year for severe algal spot infections. In the year after and going forward, the rate should be changed to two applications. Additionally, an application for an algal spot infection detected in late summer will help protect trees in the next year’s season.
CONTROL
Algal spot is associated with tree stress and is more frequent and severe on stressed or declining trees, plant pathologist Megan Dewdney of the Citrus Research and Education Center reported. She recommended reducing tree stress through cultural practices, including optimizing nutrition and irrigation, and through disease and pest management for other problems.
Trials that Dewdney and associates have conducted in recent years showed that phosphite products offered the most consistent control for algal spot over three years. Dewdney recommended three applications of phosphite products if the problem is severe in the first year. In following years, two applications will be sufficient. She said if a problem is detected in late summer, one application will help for the next year.
