Take a picture of the tree and any visible leaf damage.
Remove any spotted, damaged, dead or dying leaves from the tree and the surrounding ground. This will redirect the tree's energy to growing healthy leaves and stop the spread of most fungi and mold varieties. Put them in a plastic garbage bag and move them far from the tree. But before throwing the bag away (do not compost its contents), try to diagnose the problem yourself and then save a few to bring to your local garden center along with the pictures you took in step one.
If your leaves are covered in what looks like black soot, this is sooty mold and it develops on the droppings of aphids, whiteflies, blackflies and other pests. The mold is not dangerous, but it does keep the leaves from getting sunlight and carbon dioxide. Simply wash it off with a warm rag covered in soapy water. However, it will continue to recur as long as the pests are present.
Did you notice that your citrus tree's leaves became cupped or curled shortly after they emerged? The culprit is likely aphids, and while their mere presence on citrus trees is rarely a problem, their droppings can cause black sooty mold to develop on the leaves. If the cupping or curling is accompanied by browning leaves, then your tree may be suffering from wind burn. This will not seriously damage your tree, but you may consider erecting a windbreak to protect it.
Are your citrus leaves yellowing? Yellowed leaves are most likely an environmental problem. If the yellowing is accompanied by tipburn, browning of the leaves and considerable leaf drop then your problem is likely high levels of salt in the soil. This may be naturally occurring or due to improper fertilization.
If the yellowing only occurs around the veins of the leaves and you notice that small branches on your tree are dying, then your problem may be caused by standing water, root damage or foot rot.
If your leaves are covered in silvery scratches, your tree is likely infested with spider mites. Small numbers of spider mites are not a problem. However, a large infestation can lead to extensive leaf drop.
Are the leaves spotted? Spotty leaves can be caused by a number of problems. Irregular, oily-looking spots on the leaves in summer mean greasy spot fungus. Raised black and tarry looking spots are likely only a bit of harmless sunburn.
If the spots can be brushed off with your hand, then the spots are not spots at all, but scale insects. Do not remove these leaves. Scale insects are not a serious problem unless the infestation is extensive.
The appearance of scales under the leaves, are actually adult whiteflies. Their presence is not normally a problem, but the droppings of large infestations can cause sooty mold to develop on the leaves.
Are there black spots or spirals on the leaves? The black spots are adult blackflies and the spirals are their eggs. They are rarely ever a serious problem but the droppings of large colonies can lead to the development of sooty mold on the leaves.