Algal spot, a disease that was rarely observed in Florida citrus groves prior to 2015, has been increasingly problematic in recent years, according to speakers from the University of Florida Institute of Food and Agricultural Sciences . Megan Dewdney, associate professor of plant pathology at the Citrus Research and Education Center in Lake Alfred, warned that the disease is expected to worsen as average temperatures continue to rise.
Multi-county citrus Extension agent, Mongi Zekri, based in Southwest Florida, kicked off the presentation by providing a comprehensive overview of algal spot symptoms. Zekri highlighted that this disease is predominantly observed in sweet oranges, primarily affecting the branches. The symptoms are most noticeable during the period from June to September when the causal green alga produces fruiting bodies.
When the algal spot lesion appears, one of the first signs is the thickening of branches around the affected area. If the conditions are conducive, this disease has the potential to kill branches that are up to 2 inches in diameter. It can also stunt the growth of branches, resulting in leaves that turn yellow (chlorotic) and eventually fall off.
According to Zekri, symptoms on fruit should not be a cause for concern as they typically appear later on overripe fruit. These symptoms are characterized by dark black lesions that are circular to irregular in shape.
According to Dewdney, it is challenging to detect early algal spot infection, as it is not visible to the naked eye. While this disease has been a minor issue for most cultivars, it particularly affects lemons and limes.
Algal spot, as reported by Dewdney, is often found in trees that are stressed or declining. To mitigate this issue, Dewdney suggests implementing cultural practices that optimize nutrition and irrigation to reduce tree stress. Additionally, effective disease and pest management for other problems can also help alleviate the severity of algal spot.
In recent years, Dewdney and her team have conducted trials that have revealed the most effective control for algal spot: phosphite products. These trials have demonstrated consistent results over a period of three years. Based on their findings, Dewdney recommends three applications of phosphite products in the first year for severe cases of algal spot. In subsequent years, two applications will suffice. However, if the problem is detected in late summer, a single application will suffice for the following year’s prevention.