Answer: You do have a good mystery here. You may need to go back to the crime scene and search for more evidence, but let's examine the clues that are apparent. With all 12 acting the same, we can be pretty sure it's not just a "freaky" act of nature, where only one or two may have "mutated." And since the 12 did behave well the first three years, and now ALL are "imposters," we need to consider what environmental changes there might have been this last year. Since they are along a walkway, my first question would normally be regarding the use of salt for ice. But in Marietta, you probably haven't used any salt, right? Was there some other chemical you used along the walk or nearby that may have penetrated the area stunting these victims?
Another possibility is that you are seeing offspring of the original plants. Many hosta cultivars don't reproduce "true to type" when grown from seed. That is, the seedlings may revert back to a very different form. Can you think of anything that could have killed the parent plant (such as an herbicide) but that left the seeds to sprout the following spring?
What are the other plantings in the area? Have they grown considerably over the last 3 years to totally shade these hostas? Hostas love shade, but some variegated varieties need a little sun. If a nearby tree or shrub or other plantings have grown so as to cut their exposure, then that could be why. Of course, just the opposite could also be true, although symptoms of too much sun are usually fairly apparent with bleached out leaves.
Did you apply fresh mulch last year that wasn't aged? Fresh manure or newly chipped trees that haven't aged can cause a severe lack of nitrogen which could appear as the symptoms you describe. Also check to see that you are not smothering the plants with too much mulch. Give the crowns plenty of room to breathe.
The last consideration is to think back and see if you first noticed this on April 1-- All Fool's Day?then I'd go looking for clues among those near and dear to you!
Hopefully this will solve the mystery. If not, check for more clues, so we can find out what's going on with your hostas!
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