Jane magnolias, sometimes referred to in the converse as "magnolia janes," are popular, hardy trees that can acquire a number of infections without letting those infections ultimately get the best of them. They can suffer aesthetically from them, and diseases can shorten their lifespan.
Leaf spots are a result of fungal and bacterial infections. They begin as small, dark, discolored spots. Left unchecked, they will enlarge and may develop yellow rings around the black spots while the centers rot out. The ultimate result is leaf fall and stunted, disfigured leaves. Remove affected foliage using sterile pruning shears and dispose of the debris in a garbage bag or by burning. Rake fallen leaves and leaf matter out from under the tree to prevent reinfection, and continue to monitor the tree and remove any additional spotted leaves as soon as you see them until your tree is pristine once more.
Cankers are bacterial infections that cause swelling and bulging knots to form on branches. These corky growths can eventually grow to the point that they girdle and kill entire branches if you do not remove them. Remove cankers using sterile techniques during cool weather when the infecting organism is dormant. If necessary, you can treat the tree with a pesticide to ward off additional infections.
Verticillium wilt can create a similar look to root rot, in that the leaves of the tree will start crumpling and wilting around the edges despite the presence of necessary water. Left unchecked, the leaves will turn brown and die while the fungus works its way through the root system. If you scrape off the outer layer of bark, infected wood will be stained a dark olive green. Remove all affected branches and leaves using sterile pruning techniques, and rake out all debris from under the magnolia. Dispose of it in plastic bags to prevent reinfection. Your tree may recover if you fertilize it with a high-potassium, low-nitrogen fertilizer if the invasion was not extensive. If you spot further, persistent signs of infection, remove the plant to prevent the spread of the disease.