Q.Large Black Spots On Schefflera Plant
how do I treat this?

Certified GKH Gardening Expert
It could be a fungal leaf spot and blight such as Alternaria panax. Remove infected leaves and discard. You can use a fungicide to protect leaves that have not been infected. Check the fungicide label for the disease or for fungal leaf spots. Even more important is avoiding moisture on the leaves from overhead watering or misting. Water the soil but not the leaves. Allow the soil to dry before watering. Also make sure there is plenty of air circulation around the plant and it is not in a crowded situation.
https://extension.psu.edu/schefflera-brassaia-diseases
https://www.gardeningknowhow.com/houseplants/schefflera/schefflera-plant-care.htm