Answer: Based on your description there are several possible explanations. One is that there is some early season frost damage that does not become apparent until the heat of summer when the plants begin to stress and the internal circulation system is required to work at full capacity. And, if you tend to have oscillating temperatures in the spring you might consider covering them with a frost blanket (sold at garden centers) if there is the threat of frost after they have begun to grow.
Another is that the plants are in too much sun and the heat begins to take its toll, along with the typical drier weather. (Some varieties are more sun resistant than others. The blue hostas for example need deep shade, while some of the gold hostas are able to handle up to a full morning of direct sun.)
Another is that they need more water; the soil should be kept evenly moist (not saturated, but damp). If you have sandy soil, you may need to water twice a week during dry spells. When you water, make sure it soaks down deep. To know if you need to water, dig into the soil with your finger. If it is still damp, don't water yet. After watering slowly and thoroughly, wait a few hours and then dig down to see how far the water went; sometimes it is surprising.
Using an organic mulch can help in keeping the soil moist longer in summer, it also helps feed the soil as it breaks down slowly over time. Using ample organic matter in the soil preparation phase can also help, as this enables a sandy soil to hold moisture longer.
Finally, there are also some diseases that can affect hosta and cause foliage problems, several fungal problems can occur during warm weather especially if the foliage is kept damp. Keeping the mulch several inches away from the stems and avoiding overhead watering and avoiding watering in the evening can help.
You might want to take some samples to your county extension for a more specific diagnosis, in case it is a disease problem. If it is something that requires a chemical control, they will have the most up to date information on what to use and how/when is best to use it. I hope this helps.
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