Afflicted Tree

At the risk of turning into Derrick from BBC3's excellent Ideal series (for the non-Ideal watchers, Derrick is a boring gardening allottment bloke, who talks (even more) endless, tedium about vegetables and his allotment when stoned)  

Here at Canine Feminist Collective HQ we have two Japanese Acer Trees - a red one, and a yellowy one. Yellowy one appears to have..........measles. Behind the cut  are some photos - hippy reckons the spots are some sort of creature, they are soft and when removed look like half a grape inside. A woodlice with no leggys or facial features.

I reckon its some sort of mould. 

Trees are just wee little fellas, kept in containers, well out of the way of Slippy Dog Shrapnel.

Whatever it is - its making Yellowy Tree poorly bad

sheff_dogsgrega ?  Anyone?

poorly bad acer tree

I think they are Scale Bugs (not the correct latin term I fear)...they drink the sap I think....they are a fairly common garden pest...
Scale insects - common on maples and tomatoes, anything with lots of tasty sap. Now's the time of year when they start to be noticeable.

Hard to eradicate - they're varnished into place (a relative of them is what gives us shellac) so spraying them doesn't really get through. Your best approach is strict quarantining and picking them off one-by-one. Supposedly you can spray the young ones, but once they've turned darker coloured they're almost impervious.

Fortunately you haven't got many! "Shellac" comes from the "lac" beetle, the critter involved. This name is thought to come from the Hindi word "lakh", meaning "ten thousand" -- roughly the number per twig in a heavy infestation.
I agree with everyone else, it's scale insect.

I get it on my potted bays and rub it off by manually. It's the sort of thing you can do as and when you are in the garden for a few minutes. I don't know about Acers, but on bays I find the scales by the ribs of the leaves as well as on the younger bark.

And Grega's point about ants is important. Ants can farm scale insects as well as aphids, if that's going on you need to sort the ants out because they will keep replacing all the scales you destroy. You don't have to kill the ants, just make sure they can't get at the tree. The easiest method for things in pots (as long as you make sure the pots are still stable in relation to slippy dog etc)is to make moats. Get a suitable plant pot saucer, fill it with water, put three 'feet' in and put the pot on top so it's not actually in the water. As you are watering the pot anyway it's easy enaough to keep topped up. Stops slugs too.
Thanks everyone - Yellowy Acer has been quarantined to a lonley part of the garden with just a sycamore and the builders rubbish for company. Have explained to Yellowy Acer that even nice trees from good homes can get nits.

hippy has volunteered to patiently pick off the invaders - he has a less dramatic approach than me ("Burn It!") but if they touch my won't be pretty, I can tell you.

Large plant saucers will have to be located - the pots are quite big.

In other garden news - the Japanese Knotweed is looking very frail this year, tiny shoots - this is year 3 of chemical warfare - and it looks like we might win.

Good luck with the knotweed....its evil stuff...Ivy is the thing I have to do battle with every year its not even ours it comes in from next door....
It grows all through the stone wall between our gardens. You think you have killed it because you have destroyed every bit you can see.....HA it lurks in the wall and surges into growth as soon as you back is turned.

And it grows into my apple cordons, nasty twisty stuff.
Only saving grace ... the Sparrows love the while they decline elsewhere we have the National Collection of Sparrows in our garden....