Answer: If the moldy substance you're finding is black and powdery, it's called sooty mold. It's actually just on the surface of the leaves and you can scrape it off with your fingernail. Sooty mold is caused by insect feeding (such as scale insects or aphids). As these pests feed they excrete a sticky substance called honeydew. It is high enough in sugar content that it attracts fungal or bacterial spores, which turns it black. In addition to this honeydew, the insects can cause yellowing leaves. Check to see if that's what's causing the problem by carefully inspecting your plant for signs of scale insects (look like bumps on stems). If you find the pests you can control them with horticultural oil sprays. Since boxwoods are evergreen, they eventually lose their oldest leaves and replace them with healthy new leaves so the yellowing leaves may or may not be associated with the sooty mold that you're seeing. More investigation will help you determine the cause on your boxwoods.
Best wishes with your boxwoods.
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