Algal leaf spot, also known as green scurf, is commonly found on thick-leaved, evergreen trees and shrubs such as magnolias and camellias. It is in the genus Cephaleuros and happens to be one of the only plant parasitic algae found in the United States. Although commonly found on magnolias and camellias, algal leaf spot has a host range of more than 200 species including Indian hawthorn, holly, and even guava in tropical climates. Algal leaf spot thrives in hot and humid conditions, so it can be found in the Florida Panhandle nearly year round and will be very prevalent after all the rain we’ve had lately.
Algal leaf spot is usually found on plant leaves, but it can also affect stems, branches, and fruit. The leaf spots are generally circular in shape with wavy or feathered edges and are raised from the leaf surface. The color of the spots ranges from light green to gray to brown. In the summer, the spots will become more pronounced and reddish, spore-producing structures will develop. In severe cases, leaves will yellow and drop from the plant.
The algae can move to the stems and branches in more extreme cases. The algae can infect the stems and branches by entering through a small crack or crevice in the bark. The bark in that area cracks as a canker forms that eventually can girdle the branch, killing it.
In most cases, algal leaf spot is only an aesthetic issue. If only a few leaves are affected, then they can just be removed by hand. \It is important that symptomatic leaves are discarded or composted offsite instead of being left in the mulched area around the trees or shrubs. If symptomatic leaves are left in the same general area then irrigation or rain water can splash the algal spores on healthy leaves and branches. Infected branches can also be removed and pruned.
Preventative measures are recommended for long-term management of algal leaf spot. Growing conditions can be improved by making sure that plants receive the recommended amount of sunlight, water, and fertilizer. Additionally, air circulation around affected plants can be increased by selectively pruning some branches and removing or thinning out nearby shrubs and trees. It is also important to avoid overhead irrigation whenever possible.
Fungicide application may be necessary in severe cases. Copper fungicides such as Southern Ag Liquid Copper Fungicide, Monterey Liqui-Cop Fungicide Concentrate, and Bonide Liquid Copper Fungicide are recommended. Copper may need to be sprayed every 2 weeks if wet conditions persist.
Algal leaf spot isn’t a major pathogen of shrubs and trees, but it can cause significant damage if left untreated. The first step to management is accurate identification of the problem. If you have any uncertainty, feel free to contact your local Extension Office and ask for the Master Gardener Help Desk or your County Horticulture Agent.