Leaf Spots on Southern Magnolia

The Southern magnolia or Magnolia grandiflora is a warm climate, flowering evergreen tree that produces fragrant white flowers. Leaf spots on the foliage of Southern magnolia are an indicator of fungal or algal leaf spot diseases. These diseases can cause damage to foliage, branches and flower production. Identifying the cause for the leaf spot and early control of the pathogen is vital for treating these diseases.

Algal Leaf Spot

Algal leaf spot or green scurf is a leaf spot disease of southern magnolia, caused by a parasitic alga known as Cephaleuros virescens. The disease occurs in southeastern states in the United States and favors warm, humid climate conditions for infection. Symptoms of infection consist of one-half inch gray to green leaf spots, leaf yellowing, leaf spot and localized leaf tissue death where leaf spot symptoms occur, reports the University of Arkansas Division of Agriculture Cooperative Extension Service. Mild infections produce superficial symptoms that rarely cause serious tree injury. However, chemical fungicides are an effective method of control for severe infections that threaten tree vitality. Destroy fallen infected leaves to reduce the spread of the disease.

Phyllosticta Leaf Spot

Phyllosticta leaf spot is a minor fungal disease of southern magnolia, caused by the fungus Phyllosticta magnoliae. The fungus produces small, purple to black lesions that measure approximately .78 inch in diameter, reports the University of Florida IFAS Extension. The disease favors warm weather conditions and free moisture on leaf surfaces for infection. Prevent infection by increasing airflow by pruning large branches and avoid excess water buildup on foliage when watering southern magnolia. Copper-based fungicides are an effective method of control for severe infections.

Septoria Leaf Spot

Septoria leaf spot is a fungal disease of southern magnolia foliage that occurs in temperate between 68 and 77 degrees F. Symptoms of infection consist of circular spots on leaf surfaces that have a gray color and dark boarder, reduced vigor and plant death. Like most fungal infections, Septoria fungi favor free moisture on leaf surfaces for infection. Water magnolia in the morning so that the water has a chance to evaporate, which reduces the likelihood of fungal infections. To prevent infection, remove susceptible weeds grown nearby, plant disease resistant magnolia and reduce free moisture on leaf surfaces by providing good air circulation. Fungicides are an effective method of preventing and controlling the disease.

Sources:

http://www.uaex.edu/Other_Areas/publications/PDF/FSA-7553.pdf

http://www.mrec.ifas.ufl.edu/fdacs/Cir167.htm

http://basineducation.uwex.edu/woodland/OWW/Pubs/MISC/silvics_manual/volume_2/magnolia/grandiflora.htm