The tree is in Toronto in sandy loamy soil, twenty years old large. It is in sun because it stands alone. Can you advise re an arborist?
The fungus is not on the trunk or leaves but on the smaller branches.
Thank you for contacting the Toronto Master Gardeners.
The number one pest that affects Magnolia is scale. Scale insects suck the sap from the limbs of magnolia trees and excrete a sweet, sticky substance called honeydew that drops on everything below, attracting wasps, bees and flies. A fungus, called sooty mould, grows on the honeydew, turning everything black, which is unsightly.
The actual scale can be 13mm in diameter, they are brown and hemispherical and are covered with a layer of wax. A tree under heavy attack will be encrusted by white nodules, which is probably what you have noticed. If left untreated, the infestation could kill the tree. Scale are resilient because they are covered by this waxy substance. The application of dormant oils can be useful in controlling these insects.
It is the Toronto Master Gardeners policy not to recommend any particular arborists. However, you should be able to find an appropriate expert by consulting the following websites:
- Landscape Ontario, which has contact information for tree specialists and arborists; and
- the Ontario chapter of the International Society of Arboriculture (ISA). The ISA website allows you to search for a certified arborist and the site also includes several Online & Downloadable Resources.
I am including the links below if you would like to read further about this.