Has it been drier than normal where you are? IME, these are pretty thirsty trees and maybe just relying on rainfall this spring (which hasn't been overly abundant in some area) hasn't been enough. I had a weeping katsura in my old garden I seemingly just couldn't supply with enough water. Every summer it would dry up and drop its leaves only to leaf out again in a couple of weeks.

There seems to be a lot of mulch around the trunk....it's not planted too deeply, is it? That could certainly account for a decline and loss of or low production of foliage.

Hi Gardengal. I recently applied the mulch and I don't think it was planted too deep. Perhaps, lack of rainfall and water may be the culprit. I'm located on the north coast of Oregon and it has been drier than normal. I will irrigate more frequently and deeply. Thank you for your comments and suggestions.

Timing could suggest it was the mulch.

http://blogs.extension.org/gardenprofessors/2011/08/08/toxic-mulch-when-shredded-bark-goes-bad/#comment-878303

I doubt it's the mulch as I recently applied. I'm more inclined to think lack of moisture as the Katura exhibited the condition prior. Could be the same lack of moisture that has afflicted my viburnum that I recently posted. Weird weather and more, I suspect, to be expected in the future. I tend to think our plants are indicators of things to come.

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

Vapors from the mulch would only damage the lower leaves

Not insisting it was the mulch in this case - especially since the damage preceded the mulching - but FWIW notice that if you read the entire piece linked to a commercial (as I remember it) planting of yews mentioned had severe problems. Presumably these were of some size.

Blackening like is shown on the katsura may often be bacterial in origin.

My neighbor has a beauty. It's planted next to a pond where all his downspouts go.
mike
http://i75.photobucket.com/albums/i283/Botann/IMG_2064.jpg

Thanks Mike for sharing that beautiful Katsura. Mine needs a little TLC and maybe someday it will be as nice as your neighbors.