kubali

when an aphid, or scale insect feeds, it gives off a secretion from its backside, this secretion is a food source for an innocuous fungus called "Sooty Mold", (the black soot to which you refer..), it actually is doing no harm to the tree, save aesthetics, but it is telling you that you have an insect problem that is doing harm. Citrus needs to be on a spraying program for Fungal problems and Insect problems or eventual decline will result; Copper is sprayed once every 3 months for Fungal prevention and a Horticultural Oil every months for insect control. Fertilize your Citrus 3 times a year with a well balanced Citrus fertilizer and maintain proper irrigation during the dry season. If you spray the tree now with the oil, you will eradicate the insects, thereby removing the source of food for the mold; the oil will also loosen the existing mold, so that you can wash it off, along with the dead insects. Always follow the instructions on the back of your products (oil and copper) and avoid spraying the oil during the heat of the day, apply in the late afternoon, let it work overnight, and first thing the next morning, wash off the application and dead insects. And it is hard to say without having been there and seen it but like I said you have to water in fruit deep to keep it from aborting a lot of fruit like march thru June when fruit is developing and we usually don't get much rain here......