Answer: I can't really diagnose the problem without inspecting your tree so we'll have to try to figure it out through a series of deductions. Wet weather can encourage all kinds of fungal growths, but it can also make the soil soggy. Exceptional water uptake can cause the vascular system of your tree to become engorged and as a result the tree can ooze sap. Is the substance hard or fluffly? Hard substances could be sap escaping from the tree, or lichen growing on the bark due to the cool, wet weather; soft substances could be powdery mildew or other fungal growth. You may have to take a wait and see stance until things dry up and the weather gets warmer. The substance may disappear all by itself. If not you might want to scrape some off and take it to your local cooperative extension office for microscopic inspection. Helpful folks there can identify the substance and suggest control remedies. Good luck with your magnolia!
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