I have a very large, very old crown of thorns in an outside planter. It has done well for a long time then all of the sudden leaves turned yellow and fell off. On inspection I found tiny white dots w/black centers . They looked like tiny dirty burst bubbles. Now I'm starting to see them on other crown of thorns but so far not on the 20 or so different kinds of plants that are in close proximity. What is it and how do I get rid of it ??