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One prominent sign that the leaf drop is due to some type of strain to the tree, is the fact that most all the leaves are falling from the tree without their petiole still attached. One of the symptoms of citrus greening is yellow mottling of the leaves. If it is greening, you can find out by taking samples to your local extension agent. - Millet
As long as the grounds drain well, it has nothing to do with being too much water. Why are you aggressively spraying so much on newly planted trees? Are your trees in full sun? How and what time of the day are you spraying the oil?
Too much water, i have had the same problem this years.