Q: What are the “white spots” on my Japanese maple leaves? Just noticed them this week.
A: It’s a fungal disease called phyllosticta. It is unsightly but is rarely a big problem on Japanese maples.
Only under certain combinations of temperature and humidity will it appear. In the future, use slow-release, organic fertilizers so there is not so much lush growth at one time in spring.
My guess is that a couple of sprays of propiconazole (click for sources) would control it.