Certified GKH Gardening Expert
This can happen for a number of reasons. Fungus, pests or watering issues are the most common. Treat the tree with a fungicide and a pesticide to take care of the first two possible issues. For the watering issue, make sure the tree is getting at least 2'-4' of water a week. But also make sure it is not in standing or overly wet soil, as this can cause root rot, which makes it difficult for the tree to take up water.