Answer: Drought stress can cause dieback so I suspect that's what the problem is. Since they are already under stress, withhold fertilizer but water them deeply once each week. If you haven't already, make watering wells or basins beneath each by mounding up a few inches of soil in a circle around the trunk, about 12" away from the trunk. Fill the basin with water, allow to drain, then fill it a second time. This method of watering concentrates the water directly over the rootmass and allows it to trickle down, wetting the entire root area. Once each week should do it.
Hope your plants respond favorable in a short time!!
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