Certified GKH Gardening Expert
It is not sooty mold, but it is a severe insect infestation. It looks more like a type of aphid causing this, though. There are many natural remedies that you can use to clear this up. One of my favorites is neem oil.
Here is a collection of articles that will help you get rid of this problem: https://www.gardeningknowhow.com/special/organic/