see if it wipes off easily,.. this isn't WPM I don't think, it looks kinda like watery bird turd splats! ...texture is hard to tell, closer up pics would help if possible, but it doesn't look powdery to me, more like thin paint? ... treat anyway if he likes... I forgot to mention there are some biological agents, foliar sprays that have microbes that eat mildew, spores and all,.... Actinovate and Armory come to mind....