Click for larger image Algal leaf spot on camellia. Note fuzzy edges of lesion. H. Thornton, UGA,

Algal leaf spot is a foliar disease most commonly seen in warm humid climates or in greenhouses. The causal organism is Cephaleuros virescens, a green parasitic alga whose usual hosts are plants with leathery leaves such as cotoneasters, magnolias, hollies, rhododendrons and viburnums. Algal leaf spot is sometimes called green scurf because the spots may have a crusty, fuzzy or flaky appearance.

Symptoms and Diagnosis

Algal leaf spot is characterized by grayish, green, brown or orange cushion-like blotches on the leaf surface. Some hosts may also have diseased twigs and branches that are girdled and stunted with reddish brown fruiting bodies. The spots are generally 1/2 inch or less in diameter although they may coalesce to form larger colonies. Leaf tissue may die beneath the spots and the leaves may yellow and drop prematurely.

Life Cycle

During wet weather, the algae produce spores that are spread by wind and splashing rain. The spores infect leaf tissue causing small, greenish circular spots that may age to light brown or reddish brown. The spots may appear raised and velvety. The algae will overwinter or survive other unfavorable environmental conditions in leaf spots including those on fallen leaves.

Integrated Pest Management Strategies

1. Encourage healthy plants with good cultural techniques. This disease is most damaging on plants that are already slow-growing or weakened.

2. Clean up fallen leaves and remove diseased leaves from the plant. Good sanitation practices will help control this disease.

3. Promote dry leaves by improving air circulation and drainage. If necessary, selectively prune overcrowded vegetation. Avoid spraying water on the leaves.

4. Use fungicidal sprays containing copper if chemical control is necessary

Organic Strategies

Strategies 1, 2, and 3 are strictly organic approaches. For an organic approach to Strategy 4, consult the Organic Materials Review Institute (OMRI™) for appropriate organic copper products.

More images:

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Close-up of algal leaf spot on camellia. Note cushion-like appearance of lesion. H. Thornton, UGA,
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Red fruiting bodies of green alga on Southern magnolia (Magnolia grandiflora). FL DPI Archive, FDACS,
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Green alga on sycamore twig (Sycamore) . FL DPI Archive, FDACS,