Many/most of my crotons are suffering leaf drop right now. This has been going on since September and now many of them have gotten very, very thin and sparse. I can't blame it on cold. What is it? I don't have any croton scale in my garden and very few plants have a few spider mites. What could it be? I pick up every leaf that drops, lest it have some disease I can't see. Some plants drop like crazy, then there will be another right next to it that barely drops any. Last year the same thing happened this time of year, but not quite as bad, I don't think. I haven't a clue. I need HELP! What should I do?