Shocked to find white powdery sticky stuff on some of my succulents cactis. They are on a wooden patio and I see most of this havoc on cactis that are near the AC which was on constantly day and night.

I think the heat generated from AC did it and maybe I overwatered the plants. I have washed some with hot water and soap and they are drying ,some I threw away.NOW WHAT?

Please tell me if there is any spray I can use on plants to avoid this problem . Thanks .....Sue in LA