Algal Leaf Spot: Atlanta Diagnosis & Treatment Guide

What Is Algal Leaf Spot?

Algal leaf spot is a plant disease caused by a parasitic green alga, Cephaleuros virescens, sometimes called green scurf. It is the only plant-parasitic alga common in the United States, and it produces raised greenish, gray, or red-brown spots on the leaves, and sometimes the twigs, branches, and fruit, of broadleaf evergreens and other landscape plants in the Southeast.

How to Recognize It

• Circular or blotchy raised spots on leaves with wavy or feathered edges, typically 1/4 to 1/2 inch across.
• Spot color shifts from crusty gray-green or greenish-brown to a velvety red-brown or rust-orange in summer, when the alga produces its reproductive structures.
• Heavy infections cause premature yellowing and leaf drop.
• Spots can also appear on twigs, branches, and occasionally fruit, with bark thickening and cracking where the alga has invaded.
• On older spots, grayish-white patches may form when a fungus colonizes the alga and creates a lichen (Strigula), which is itself parasitic on the leaf.
• Damage is most serious on twigs and branches, where cankers can form and eventually girdle and kill the branch.

Symptoms become most noticeable in summer when the alga produces its red-brown, spore-bearing structures, and spores spread during warm, wet weather through rain, overhead irrigation, and wind. Winter is a good time to inspect evergreens like Southern magnolia and remove fallen, infected leaves before warm weather returns.

Why It Matters for Atlanta Trees

Algal leaf spot is usually a cosmetic problem on otherwise healthy trees and rarely causes serious decline on its own, though it tends to be worse on plants already weakened by poor growing conditions. The more important risk is on twigs and branches, where the alga can create cankers that girdle and kill branches over time, so persistent or branch-level infections are worth a closer look.

In Atlanta landscapes, the disease shows up most often on Southern magnolia, sweetbay magnolia, camellia, American holly and other hollies, Indian hawthorn, sycamore, crapemyrtle, rhododendron and azalea, and other broadleaf evergreens with leathery leaves.

General Prevention

• Improve airflow and light around affected plants by selectively thinning nearby vegetation or crown-thinning the tree itself.
• Keep trees vigorous with proper watering, mulching, and fertilization, since stressed plants are more susceptible.
• Avoid overhead irrigation that keeps foliage wet, and water at the base of the plant instead.
• Rake up and dispose of fallen infected leaves, and prune out heavily infected twigs or branches during dry weather.

What NOT to Do

• Do not self-diagnose. Many tree problems look alike, and treating the wrong one wastes time and can harm the tree.
• Do not apply fungicides, insecticides, or other chemicals without an arborist's specific recommendation. Wrong product or wrong timing makes things worse.
• Do not prune symptomatic twigs or branches without sanitizing your tools between cuts, since the alga and any associated organisms can be carried from one part of the tree to another on contaminated blades.

Related Services

For most diagnosis and treatment questions, the right starting point is one of our services:

• ISA-Certified Arborist Services for diagnosis, consultation, and second opinions.
• Plant Health Care (PHC) for ongoing tree health management.
• TRAQ Tree Risk Assessment for when the tree may be a safety hazard.

Sources

This page summarizes general information from: Clemson Cooperative Extension (HGIC 2060), NC State Cooperative Extension, Craven County Center, and University of Hawaii (peer-reviewed extension PDF).