A few weeks ago I planted an Alexandrina Magnolia tree in my backyard. I've been watering it 2 times a week (plan to slow down once it's been planted for a month), it receives full sun/partial sun depending on the hour, and has mulch and soil around it. It grew fine until a couple of days ago when I noticed brown/yellow spots on the petals. They looked almost withered/wrinkled, and I was worried it might be some kind of infection. I've plucked off most of the affected petals but I don't want to harm the tree any further. Can anyone help me identify what kind of infection/disease it has (if it does) and how I can treat it?