For the last few years, my otherwise healthy hostas have turned brown particularly around the edges by mid-summer. The blue ones seem unaffected by this blight. I'm thinking it's anthracnose. Last year, I eradicated slugs from the bed with Sluggo and haven't seen any since. If it is anthracnose, how can I treat this? I have numerous plantings so I don't want to get rid of them. Also, if I use copper fungicide, how frequent should I apply as I don't want to harm earth worms.
It may be anthracnose. We'd recommend cleaning up all dead foliage when it die back in cold temperatures, aiming for garden sanitation, rather than using a fungicide.
Do not water overhead.
Edge browning of leaves can indicate lack of water (and/or root dieback from drought). Water during prolonged dry periods.
I'm fairly certain that it's not drought, as I have had the same experience for several years including last year with our record rainfall. The hostas are all in part to full shade locations. I have removed the old foliage each fall to no avail and need an alternative solution.
It's too late in the season to spray a fungicide now. You can spray beginning when foliage appears in spring next year, but you'll have to spray repeatedly over the growing season. We'd recommend replacing with a more disease-resistant variety if possible.