Ask-a-Dave's-Gardener: Cassia Fistula brown yellow/brown leaves, 2 by sartex

sartex wrote:
Hello everyone, I am new to this site and would like to say thanks first since I have come here for valuable information in the past, I was wondering if anybody here would help me identifying what\'s wrong with my tree, this Cassia I bought it a few years back from top tropicals nursery, the young tree around spring starts with new growth but when fully mature, the leaves never turn the dark green they are supposed to be, I have tried measuring the soil PH( found to be neutral to acidic), I have tried epsom salts which helped a bit and ultimately ironite but the problem persists, the laves never reach to be dark green and then they all get brownish and like burnt at the edges, this happens every year, I wonder if I have a any pest killing my trees as the pictures also shows a dying totally defoliated cassia javanica in the background which I finally gave up with...I appreciate any advise from anyone here. Thank you.