Those are some challenging plant issues. The mango seems to be suffering from a root problem. It could be dry soil, burn from salt based fertilizers or physical damage to the roots. Since one tree is okay and the other is not and I suspect your care of them is similar I am at a loss as to why one is showing the problem. One possibility is that the planting hole of the first tree was glazed to create a problem with roots venturing out from the small, confined area. Thus the leaf symptoms showing lack of water. The only other suggestion is to consider how the particular planting sites and care might be different.
The plumbagos may have been suffering from soil moisture extremes or possibly nematodes in the soil. Your County Extension Office may be able to assist you in diagnosing these plant problems. With a sample in hand or at least some photos they would better be able to determine the specific causes. Good luck in getting to the bottom of this and sorry we could not be more specific in our assistance this time.
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