I mentioned the supposedly superior cold hardiness of Korean Stewartia to the Dulith, MN-based nurseryman I bought my S. pseudocamellia. I suspect he had heard this before, and he became almost agitated when I mentioned it. I have no idea if one is hardier than the other, but they are both hardy in z4. Mine has seen -25F a few times. It survived the infamous 2013-‘14 winter with no visible damage whatsoever, but flowering was very sparse that season.

Wet out there this morning. We got a normal month's amount of rain in one day Tuesday This grove is about the size of a city lot. I originally planted them out to sell on about 4 ft centers. An employee hacked them in half at about 6 ft. and completely ruined them. I left them alone and they grew a lot larger. They were planted too close to a row of Thuja plicata and as they got larger, started to lean. Then an ice storm leaned them over more. I finally got in there with a chainsaw and thinned them. Every stump sprouted new growth. I made the problem worse! So I cut all those sprouts off and when they sprouted, I hit em' with a broad leaf weed killer. This is what's left. All the trees are Katsuras. Mike

I'm inclined to think that lack of water is the culprit rather than vapors from anaerobic mulch because the leaves are affected all the way to the top of the tree. Vapors from the mulch would only damage the lower leaves. A garden maintenance acquaintance of mine dropped off some plastic bags of lawn clippings that smelled real bad because of no oxygen. I spread them out in a nursery bed I had full of azaleas. It almost killed them overnight. I removed the mulch and watered the plants real good and they slowly recovered. The bottom leaves showed more damage than the tops of the plants as shown in your link, Ron. I thought the clippings contained fresh broad leaf weed killer and confronted my friend. He said no, there was no weed killer involved. I was stumped as to the cause....until now. You can get the same effect by spreading Caseron and not covering it with a mulch. Mike

Probably a water related issue, such as damage from the hot summer we had last year. Or a root pathogen such as honey fungus interfering with hydration of the top by attacking and diminishing the root system. This situation may require a consultant who can visit the site and poke around in the soil in person in order for the problem to be identified. Maybe including sending samples to a lab for diagnosis.