Raised, wavy, cushion-like spots are symptoms of algal leaf spot, also known as green scurf. The spots are greenish, pinkish or grayish and less than 1 cm. in diameter. They may encircle a twig, creating a girdling canker.
Plants weakened by other stress factors are the most susceptible to infection and further weakening from this disease. It affects magnolia, camellia, holly, and many other plants.
Algal leaf spot is caused by the alga (plural, algae) Cephaleuros. Thick-leafed plants are infected in the summer, when rain spreads the pathogen. Colonies can survive on fallen leaves for a time and spread to growing leaves. After the initial infection, secondary reproduction or spread occurs over an 8- to 9-month period.
This disease primarily causes damage to the appearance of plants. Otherwise, the impact is usually minimal. When new growth is expanding, make two applications of an approved algaecide, spaced 4 weeks apart. Collect and discard infected, fallen leaves to help reduce the spread of the disease. If the infestation is limited, selectively remove infected leaves from the plant.