Bumps along stems and sticky substance on the leaves of magnolia are indications magnolia scale has infested your tree.
These insects feed on twigs and branches, removing large quantities of plant sap. They excrete the excess as a clear sticky substance called honeydew. Branches completely covered by scale insects can be weakened and even killed.
Remove small infestations by pruning out the few scale-infested stems. Treat heavier infestations with lightweight horticulture oil. This organic option will not harm our essential pollinators. Spray the tree when the immature scale, called crawlers, exit the shell-like covering to find a place to begin feeding. These tiny red crawlers usually appear between late August and early October. Be sure to read and follow label directions.
A bit more information: You can also apply horticulture oil in fall before freezing weather arrives and in early spring to kill the overwintering stage of this pest. Or spray the tree with dormant oil in late March before the buds open and when temperatures are above 40 degrees.