My large plant is losing leaves suddenly at a tremendous rate. Some if its "trunk" parts have a white kind of cotton on them and a lot of dirt sticks to the leaves.
Diseases and Pests of Dipladenia
Certified GKH Gardening Expert
It sounds like you have both a Sooty Mold issue as well as Mealybugs.
Here are links that will help you with both of these issues.