Black scale that infests magnolia trees can cause yellowing leaves, branch and stem dieback, and often leads to sooty mold disease. Sooty mold is a fungal disease that turns the leaves black with mold growth. The scale insect feeds on the magnolia tree's sap and secretes a sticky liquid, called honeydew, onto the leaves and branches. The honeydew allows the black sooty mold to grow and multiply on the magnolia tree. Heavy scale infestations and black sooty mold infections can debilitate the magnolia tree severely and, if left untreated, kill the tree.
Remove and destroy any heavily-infested branches on your magnolia tree. Clear away all fallen leaves and discard them to prevent the spread of disease. Disinfect your pruning tools before and after using them on the infested tree by dipping them in a solution of three parts denatured alcohol and one part water.
Spray the magnolia tree's leaves and stems thoroughly with a horticultural oil, or "summer oil," at 1.5 to 2 percent concentration. Perform this application in late August, when the scale "crawlers" have become well-established in the tree.
Spray the leaves and stems thoroughly with dormant oils in October or November. Apply the dormant oils again in March, before the buds begin to swell, to kill any nymphs that survived the winter season.
Treat the magnolia tree with an appropriate insecticide or insecticidal soap in late August or early September, instead of using the horticultural oils. Follow the directions on the insecticide label exactly.