Looks like the beginnings of Powdery Mildew.

Powdery Mildew - Sustainable Gardening Australia

the above link contains non poisonous controls for powdery mildew.

I haven't ever had powdery mildew on cannas, but I have treated zuccinni plants effectively with full cream milk, lightly sprayed on the plants once a week.
sarah louise Reviewed by sarah louise on . please tell me this isnt mold!!!!!!!!!! can someone look at these pics please. on some of my leaves it appears to have small white circular patches on the leaves. ive seen these patches for some time now and nothing has changed. ive rubbed on the patches and it barely came off still being there. the plants are in about 30days of vegetation. im just worried cause im almost ready to throw it in the 12/12 cycle and i dont want my bud later to turn into mushy mold. soil-ff ff nutes indoor any other info you need ask me, i Rating: 5