I have several potted Mandevillas on my terrace, and I’ve been fighting a war on mealy bugs and black spot all summer. I’ve tried everything! Even replacing all plants at the same time and washing all planting materials (pots, etc.) with alcohol. Currently I’m using Neem Oil. These little suckers are so miserable that when you cut into any part of the plant, there is no sticky sap. All my plants have been sucked dry! I have never seen this before.
Note attached picture are leaves from from two separate plants. One leaf is in the beginning stages of black spot; and the other depicts a heavy mealy bug infestation on the back of the leaf.