Jasmine

Question

Our plant leaves are covered with black blotches almost looks like dirt.

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Answer 1

This is Sooty Mold and it is actually caused by pests.
The yellowing leaves are likely another sign of a pest issue.
Treat the plant with Neem Oil, make sure you spray all parts of the plant and undersides of leaves.
Here are some links with more information.

https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm