Certified GKH Gardening Expert
If newly planted, it is probably a bit of transplant shock. I would give it some fertilizer, continue watering every day and this article will help as well: https://www.gardeningknowhow.com/plant-problems/environmental/learn-how-to-avoid-and-repair-transplant-shock-in-plants.htm
On established trees, it could be a fungus. I would treat the tree with a fungicide every 2-3 weeks for a few months. I personally like neem oil as a fungicide, as it is systemic and you do not need to be able to reach all parts of the tree in order for it to be effective.