The southern magnolia (Magnolia grandiflora) is a lovely ornamental tree that's commonly grown in landscapes located in mild-winter regions. Southern magnolias bloom in huge, showy, saucer-shaped flowers that are extremely fragrant. Although the southern magnolia tree is rarely bothered by harmful insects, it is susceptible to several bacterial and fungal diseases.
The fungal disease Verticillium wilt attacks the southern magnolia tree's vascular system, producing symptoms like faded, yellowed or browning foliage, wilted leaves and wilting, dying shoots and branches. Verticillium wilt typically causes symptoms that start on one side of the magnolia and spread to the rest of the canopy. Newly infected southern magnolias will display dark stains on the grain of infected wood when you peel back the branch bark. Verticillium wilt can kill the branches and the entire tree in severe infections. To treat this disease, prune away all the dead or dying branches and keep the southern magnolia well-watered and fertilized.
Leaf spot diseases in southern magnolia trees are typically caused by harmful fungi and are easily diagnosed. Algal leaf spot, caused by the fungus Cephaleurus virescens, creates silvery-grey, greenish or tan, raised spots on the leaves. The spots or blotches tend to have green rings or edges around them. In severe infections, the southern magnolia's leaves will turn yellow and drop from the tree.
Algal leaf spot disease occurs most often during wet, hot weather and can severely damage weakened or injured magnolias. Phyllosticta leaf spot is another fungal disease that causes yellow or brownish spots or patches on the southern magnolia's leaves. Treat a southern magnolia with leaf spot diseases by raking away and pruning off all infected leaves, and then destroying or discarding the diseased plant parts. Also avoid wetting the leaves when watering the southern magnolia tree.
Most common during extended wet conditions during spring, bacterial blight is a disease in southern magnolia trees caused by the bacterium Pseudomonas syringae. Bacterial blight can cause large, irregular, brown spots on the magnolia's leaves and blossom, or stem-tip dieback. The leaf spots may be smaller and black, and sometimes stem cankers develop. Treat bacterial blight in your southern magnolia tree by pruning away all symptomatic growth and withholding fertilizer. Avoid wetting the foliage when watering and try to prune during drier conditions to prevent the spread of infection.
Canker diseases, particularly nectria canker, can also infect southern magnolia trees, often causing areas of dead tissue on stems, branches or trunks. The diseased bark usually appears sunken and discolored, and can be surrounded by layers of callus tissues and ooze a resin-like substance. The leaves on the infected branches tend to turn yellow or brownish and wilt. Prune away the diseased leaves, twigs and branches to control canker diseases. Carving out cankers on larger branches and trunks can also help, as long as you carve out all the infected tissues with only healthy wood and bark remaining.