This is distressing to hear.
We have several magnolias and two are enormous (15ft) and I've noticed small white dots on the leaves. On Parr Drive I see that the huge magnolias are all dying and I'm hoping that I can save my trees. I'm having Megan Heichelbech of M&M Landscaping spray the trees. Megan is a talented hortilculturist so hopefully she can save them.