Growing fruit trees in the desert has a few advantages, because the low humidity and extreme conditions make it an unfavorable environment for pests, making plants less prone to many diseases
Plants have been harmed by diseases and insects since the earliest times in recorded history. To
avoid giving diseases favorable conditions, the maintenance of vigor by means of proper cultivation, irrigation, and manuring is important. It is the run down trees which are most likely to be attacked by pests.
If trees seem to lacking in health, the first move is to find out, if possible, the cause. Look  for insects or insect damage
Don't rush to use chemicals, there are many natural products you can use which are environmentally safe.