Answer: Slugs would usually eat holes in the leaves and leave behind a tell tale silvery trail. If you are not seeing the trails, then slugs are not the problem. Frost damage can cause a discoloration later in the season. But, based on your description, I think it may be sunburn. Hostas can tolerate some gentle morning sun or bright dappled light all day, but will not handle hot afternoon sun. The moister the soil, the more sun they can handle but only up to a point. If they are suffering from too much sun, you should consider transplanting them.
Q&A Library Searching Tips