Q.diseases and pests of dipladenia
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.
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.
https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm
https://www.gardeningknowhow.com/plant-problems/pests/insects/mealybugs-control.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/growing-a-dipladenia.htm