I have a few yellow orchid vines that were planted about 3 months ago and are in decline. There are three factors that I think could contribute:
1) I live in PHX and we have experienced record heat in the last two weeks. Despite that, the plants have never gone dry and are watered by drip system.
2) I've seen a white powdery substance mainly on the stems, both mature and new.
3) Many leaves show the spots as in the image.

I have sprayed with a potassium bicarbonate solution a couple of times (at night only). I've also sprayed once with neem oil.

I would appreciate if someone can help me diagnose the likely cause of the decline and a recommended treatment.