All I can tell you is that the plant is suffering from some fungal disease and the usual means of controlling a fungal disease is with the use of copper sprays. Why the plant is wilting I don't know either but the UAE does not have a tropical climate and this plant is native to Cuba a tropical country. It is also very dry in the UAE so it might be suffering from a lack of enough water. Have you fertilized them? In Florida the tree is usually fertilized 3 or 4 times per year with a high phosphorus content fertilizer, a fertilizer with a higher middle number.