Might just be sunburn.

Yes, thanks. That's the first thing I thought, but it only gets some morning sun. I have seen other Ukigumos getting much more sun that look healthier. I even cleared some things to give it more light this year, but still same thing.

Otherwise Pseudomonas seems to be pretty common on grafted Japanese maples here - this might be some manifestation of that or another bacterial pathogen. An 'Ukigumo' I was given and had here for years did the same thing, plus there was some twig death. It never made any significant progress and may have finally died on its own before being removed - I don't remember the final outcome now.

Another, much larger example I have seen at a perennials nursery in Whatcom County fared better and was total enchantment. A California man with some kind of past involvement in the nursery industry who used to post long comments on the UBC Botanical Garden Forums claimed that careless producers were selling grafted Japanese maples that were "internally" infected with Pseudomonas - with these infestations taking time to manifest conspicuously, so that plants were looking good at time of purchase but then going south after installation on the final planting site. If this is true then you have to both avoid infected specimens at time of purchase as well as planting sites that promote infection later - what it likes is a combination of immature wood and cold and damp atmospheres in fall. Around here Japanese maples and bacterial blight can be like tomato plants and late blight, with the tops deteriorating quickly in fall - when the weather turns cool and damp, enabling the pathogens to strike.

I agree it may very well be pseudomonas. There are several cultivars of JM's that seem to be overly prone to this bacterium - 'Sango Kaku', 'Corallinum', 'Oshi Bene' and 'Ukigumo' in particular.

The PNW Disease handbook has a very good overview on this pathogen, including tips to avoid infection as well as treatments to control.

Here is a link that might be useful: Pseudomonas syringae

Thanks to you both--but sounds like bad news. I don't have the heart to take it out, but if I did, should I avoid planting another JM there?

I traded mine away. I never could get it good looking in the Fall.
My friend is having the same trouble with it this year. I warned him.
Beautiful tree in the Spring though.
Mike

Pseudomonas is not really that bad........certainly not necessarily fatal. It can be treated with fungicides (see the link above) or bactericides. Often just pruning off the infected tissue (branch ends, typically) can suffice to keep the disease in check. And attention to cultural issues will help as well.

It is really your choice as to removal and there is nothing limiting replanting with another JM if you choose to do so.

If you move to another spot it might do better, unless your whole garden has the same atmosphere. Most of my girlfriend's large property is in a cold air drainage - at the right times of the year you can see mist come down the pasture across the road and float through her place, past her house to a nearby wetland - and we have had a problems with plantings of maples there. The shrubbier branch sport of 'Bloodgood' from Oregon that has a name that is escaping me at the moment didn't make it to even a third year, as I remember it.