Answer: It sounds more like an insect than a disease, but without seeing the leaves, it's difficult to say for certain. Powdery mildew, a fungal disease, can leave a white coating on leaves and cause them to curl, but it's unusual on citrus, and generally occurs on the upper surface of the leaves.
A more likely problem is cottony scale or mealy bugs. Both insects are common pests of citrus and a by-product of each is a sticky honeydew with accompanying sooty mold growth on plant parts below the infestation.
Without knowing precisely what you're dealing with, it's hard to suggest control measures. Why not take a sample of the problem to your local Cooperative Extension for diagnosis and treatment? In Orange County phone 714-708-1606 to find a plant problem clinic near your home.
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