Answer: If you've already lost 3 Champaca trees, there's obviously something wrong with the soil, with the way you've planted them, or with the care they've been receiving. If the new leaves began growing normally and then started flagging, the roots are either getting too much or too little water. Spraying insecticides can harm the leaves if the product is oil based (always make sure the pest and the plant you intend to use it on are both listed on the label because some plants are super-sensitive the the chemicals contained the the product).
In general, Champaca can be slow to establish and slow to grow. Your tree needs to be watered regularly, especially during the spring and summer months. Afternoon shade (or filtered sun) will suit your tree just fine. When you water, water deeply, providing about one-inch of water per week to the roots under the canopy of your tree. This should help provide adequate moisture for it to thrive. I'd prune the tree back now, removing the dead branches. Pruning will stimulate new growth which will improve its appearance. Wait 3-4 days after watering and dig a small hole near the roots of your tree to check the soil moisture; if it's still moist 3" beneath the surface, you won't need to water for a few more days; if it's dry, it's time to water; if the soil is soggy wet, it doesn't drain adequately and your tree can suffer root rot. It would be best to move the tree to a location with better drainage.
Best wishes with your tree!
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