Deciduous Magnolia – Powdery Mildew
Q: My tulip tree just started getting this fungus on the leaves in the past several weeks. What could it be?
A: Warm days and cool nights make great environmental conditions for powdery mildew to develop on magnolia leaves. It happens every year in early summer and again in fall.
One way to control it is to apply horticultural oil (click for sources)
see Powdery Mildew
Honestly this late in the season (October) little damage will be done to your magnolia but it’s probably a good idea to start spraying your plant preventatively next year around the end of August every couple of weeks.