Certified GKH Gardening Expert
If newly planted, it may be dead due to transplant shock. This article will help you determine if it is dead or not: https://www.gardeningknowhow.com/plant-problems/environmental/how-to-tell-if-a-plant-is-dead-and-how-to-recover-an-almost-dead-plant.htm
If it is not dead, treat it for transplant shock:
It could also be a fungus which is making the leaves turn brown. The fungus is probably just mild enough to damage the leaves but leave the tree still alive. This can be treated with a fungicide, such as neem oil.