Hello, from the picture you sent longan tree looks like potassium deficiency. Potassium deficiency: mainly manifested as yellowing and scorching of leaf tips and leaf margins toward the center, with older leaves showing symptoms first.

Remedial measures:

1, combined with the autumn application of base fertilizer (around August 20) or growing season fertilizer, increase the application of potassium sulfate, 3 to 5 kg per mu of soil.

2, growing season foliar spraying grass ash leachate 50 times liquid or potassium dihydrogen phosphate 300 times liquid, every half-month interval spraying, *** spraying 2 to 3 times.