Answer: I'd first check the plant carefully to make sure it has a fungal disease and is not covered with sooty mold. Sooty mold is black and you can wipe it off the leaf surfaces. Sooty mold is the result of aphids (or other sucking insects) feeding on the fluids within the leaves. As they feed they excrete a sticky substance called honeydew. This honeydew is high enough in sugars to attract fungal or bacterial spores. With age it turns black. If this is what is on your plant you'll want to control the insects and the problem will go away. On the other hand, if the plant has developed a true fungal or bacterial disease, it either is getting too much shade, too much water directly on the foliage, or there's not enough air circulation around the plant to keep it healthy. You can try trimming the diseased portions off the plant, which will not only get rid of the problem but will also help open it up to better air circulation.
Hope this information helps!
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