Q.black dust on leaves
It’s a black coating and will come off if scratched.
Downtoearthdigs
Answered on March 12, 2017
This is Sooty Mold, that is caused by pests.
These links will help you.
https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm
https://www.gardeningknowhow.com/ornamental/shrubs/rhododendron/rhododendron-sooty-mold.htm
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