Black spots on the leaves of my dwarf orange tree. Loads of oranges, but no leaves. Garden shop said I was overwatering, but I didn't water until early January...filled basin with water once every 10 days to two weeks. No change. I fed and mulched tree February 1. Miniature tangerine tree (about 7 feet now). Excellent fruit and green leaves, but small, curling leaves at the very top. Filled basin February and fed and mulched tree. No change.
Miniature Tangerine Tree
Certified GKH Gardening Expert
The most likely cause of the problems you see are sap-sucking pests. These can cause curling leaves, leaf loss, and the black spots you see, which are known as sooty mold. Please see these articles for advice:
I am not sure what arrangement you have with the basin you mention. If your tree is in a container and the container is in a basin that you fill up, you are watering in a way that is likely to lead to root damage and salt buildup in the soil. Instead, you should water from the top and allow the water to drain freely through the drainage holes.