Young plant/tree, white substance, branch broken, leaves falling off
We're interested in the white object at the base of the trunk. Is that a plant label? A rock? We can't quite tell from the photo if this is related to the symptoms you're asking about.
The white substance on the branch could be planthopper insects or white mold, possibly. A way to check would be to brush the substance with your fingertip and see if you there are insects that move. Planthoppers produce a waxy substance like this as a form of protection. They would not, however, cause a broken branch. To check for white mold, look in the area of the white substance to see if there are black, hard seedlike structures (call sclerotia) inside stems or on stem surfaces. These sclerotia somewhat resemble black sunflower seeds. Here is information on white mold, what to look for, and how to manage it. https://extension.umd.edu/hgic/white-mold-flowers
In addition, another thing to look for would be spider mites, which might be causing the yellow spotting on the leaves. Check both sides of the leaves for signs of tiny insects. You may need a hand lens to see them. They can be problematic in hot weather particularly. Take a look at our page about spider mite for more information on what to look for. https://extension.umd.edu/hgic/spider-mites-trees-and-shrubs