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Algal Leaf Spot of Southern Magnolia

Publication Number: IS1657
View as PDF: IS1657.pdf

Southern magnolia (Magnolia grandiflora) is susceptible to Cephaleuros virescens, a parasitic alga that causes leaf spots and twig cankers. Warm, humid weather common to Mississippi encourages the growth and spread of this pathogen.

Algal leaf spot begins as a round, green, somewhat fuzzy or velvety colony on the leaf surface. The green spot will turn reddish- brown with age. Often a fungus grows along with the alga, giving the spot a grayish appearance. The fungus is not parasitic on the magnolia nor the alga. Algal leaf spots that have been colonized by fungi are referred to as being lichenized.

The alga spreads by rain-splashed or windblown spores that are produced in wet weather. The pathogen overwinters and weathers adverse environmental conditions in twig cankers and leaf spots.

Algal leaf spot is most severe on magnolias that are weak and in poor vigor. Trees that are open-grown and subject to direct sunlight, high temperatures, and excessive leaf wetness from rain or irrigation are more likely to get the disease.

The following activities are useful in managing algal leaf spot:

  1. Maintain vigorous trees with proper watering and fertilization.
  2. Avoid irrigation systems that spray water onto leaves.
  3. Rake and destroy fallen leaves. You may also want to pick off infected leaves from trees that have very minor infections.
  4. Prune overhanging branches of surrounding plants to reduce humidity by improving air circulation.
  5. Apply a copper-containing fungicide such as Southern Ag Liquid Copper Fungicide, Monterey Liqui-Cop, or SePRO CuPRO 5000 T/N/O.

Note: Chemical labels change; always get current information about usage, and examine a current label before applying any chemical. Always follow label directions when applying chemicals.

The information given here is for educational purposes only. References to commercial products, trade names, or suppliers are made with the understanding that no endorsement is implied and that no discrimination is intended against other products or suppliers.

Information Sheet 1657 (POD-05-23)

By Clarissa Balbalian, Diagnostic Laboratory Manager, Plant Pathology.

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