There could be many reasons for it. Like pollution, exposure to climatic hazards, and also if tree lacks in nutrients and other required supplies, trees might turn like this. I recommend to add pesticide and any chemical free fertilizers to protect trees from any bacteria or insects attack. I suggest you to check with this website, https://ariesagro.com/ - if required, consult with the team to find issues in exact way. They also has released lots of nutrient and fertilizer products on their website for effective plant growth and structure.