Hello Rebekah,

It is not fungus but an insect; your Magnolia has magnolia scale or Neolecranium cornouparvum. The scale, the white bumps that you are looking at, excretes a honeydew which is mainly sap that attracts a sooty mold that turns the tops of branches and leaves black I’m attaching an MSU Extension bulletin that describes the insect. The honeydew can be removed with a strong spray of water from the hose.

https://www.canr.msu.edu/news/check_magnolias_for_magnolia_scale_now

You will see in that article that controlling the scale depends on the stage of the insect. June to early July is the time to apply systemic insecticide which is absorbed by the roots and will move from the roots into the branches.

The next stage in the insect’s life is the active crawler stage which occurs in Michigan in August through September when you can spray with a contact insecticide. You should not need to use the second application if you do the first.

Always read and follow the instructions on the label for the best result when applying pesticides.

MSU’s Enviro-weather site has a tool which states when the crawlers are active.

http://msue.anr.msu.edu/news/magnolia_scale_is_starting_to_appear_on_magnolias

Finally, since your tree is too large for you to treat, consult a certified arborist. Go to: www.treesaregood.org, the website of the International Society of Arborists to find one in your area.

Thank you for contacting Ask Extension.