Saucer magnolia sap/exudant?
Oakland County Michigan
It is likely the honeydew from scale insects that are feeding on your magnolia. These are called magnolia scale. If you would like to attach a couple pictures of any bumps you see on your branches I may be able to confirm this for you. Compare the pictures and symptoms with these links-
If it is magnolia scale, timing the treatment is important. A tool called Enviroweather uses weather monitors all over the state. The weather station data closest to you will tell you when to treat the crawlers, which is the next generation of scale and is the most vulnerable to chemical controls. Crawlers may hatch about end of July through early September, depending on how hot the summer has been. The Enviroweather tool tracks this and calculates the best date for each summer.
I checked the tool and it is too early to treat crawlers. Right now, keep the tree well watered by using a soaker hose in a ring around the root zone, or a sprinkler set very low so it dribbles without hitting the leaves, moving it several times around the root zone. Don’t drown the tree just keep the root zone moist down 8 inches or so.
It can take a couple of years repeated treatment to gain control.