Sooty mold is a common problem in magnolia trees in southern Florida. This black mold is actually a secondary symptom of an infestation of soft scales, aphids, treehoppers or other honeydew-excreting insects. In the humid Florida climate, black or sooty mold develops on the honeydew. Rain washes sooty mold away so often that in many cases it goes relatively unnoticed. However, in Florida's dry season, the black soot can build up to unsightly levels. Sooty mold is relatively harmless and easily rinsed off--but until you control the underlying insect infestation, it will keep coming back.
Inspect your magnolia tree's leaves (especially the underside), buds and branches to find the insects behind the black soot infection. Prune an infected leaf or twig and take it into your local county extension office or garden center to correctly identify the insect.
Prune any leaves or twigs that are heavily infested with insects. Snap infested leaves off of the tree with your hands. Prune twigs back to the parent branch with pruning shears. Make one clean cut and leave as little stub as possible.
Spray your magnolia with horticultural oil (a 2% mixture of a paraffinic mineral oil works well). Cover all of the plant's foliage with oil but stop just before it drips off of the tree. It will smother the insects on contact and dry out the black soot, which will eventually blow away. Scale insects can only be effectively controlled with horticultural oil in early spring when they are in their nymph stage or between August and September when they are in their crawler stage. Once in their adult form, their waxy covering will afford them some protection from the oil.