Answer: There are several possibilities I can think of for what is happening to your hostas.
One is that it is slug damage but you don't see them because they only come out at night. If it is slug damage, the holes will be eaten from the edge on inward across the leaf.
If the holes are scattered across the leaf, it may be old hail damage from earlier in the season; these holes tend to look worse as the season progresses.
Another possibility is that it is sun/moisture stress damage; if you have overfed the hostas and they have grown extra lush, they may be suffering moisture stress. When you water, be sure to water deeply about once a week rather than frequent light sprinklings.
Hostas are really not heavy feeders, so a light application of a balanced fertilizer such as 10-10-10 early in the spring should be sufficient in most cases. If you are using a foliar feed, especially if it is applied on a hot sunny day or mixed too strongly, it is possible that you have caused some fertilizer burn on the foliage.
Finally, it is possible that what you are seeing is simply the aftermath of frost damage from last spring. When a late frost nips the emerging leaves, it may not be apparent until much later in the season.
If it's just a few leaves, you can remove them carefully at the base to try to improve the appearance of the plants. Good luck with your hostas!
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