I can't necessarily tell from the pic but if you did a foliar with Epsom that's likely what it is. I see crystalline chunks which in and of itself looks peculiar. Outside of those I cannot tell if the white is salt residue from the foliar or powdery mildew.
Either way,
@Capt C is correct, you should have a good integrated pest management plan (IPM) in place. Rotating through 2-3 different approaches is considered to be best practices. I personally use Plant Therapy, Flying Skull Nuke 'Em, essential oils, and a Pyrethrin product (bomb, spray, flowers, etc)
Of course you do not need to buy a Ready-To-Use (RTU) product off of the shelf as you can combine relatively easy to acquire items to mix and make your own, if that's your thing. I don't mind sharing recipes I have on hand.