Could be cushion scale, not sure - spray with Provado, they do one that treats scale infestation, might be Ultimate Bug Killer, check the bottle, otherwise its wash off with horticultural soap, but that's not easy on soft leaves.

if it is only on a few leaves I'd be tempted to remove the whole leaf. Is it wet/frothy/dry? to the touch. If dry could be eggs from some sap sucking insect. but I'd ceratinly try a systemic insecticide as bamboo suggests.

and may i say dispose of the leaves very carefully ie maybe burn or put in a sealed bag and tie it up.

It's called velvet erineum gall mite. The spongy masses on the back of the leaves are where the mites exit the plant in their mobile phase. Not really an effective treatment except picking off effected leaves.

The scientific name is Aceria erinea

Thanks for all your help, could it spread to my other (japanese) acers Fractal? if so it has to go, where's my pruning saw?!!!! :-(((

Fractal I've googled it and I don't think it's the same thing, mine seems flatter and appears to form a continuous flattish mass, whereas on googled pics it seems to be more prominent, separate blobs if you see what I mean?

remember to sort the old leaves out or it could carry on to .

I've cut everything off I can see, it's a grafted little tree, so many leaves are affected:-(

I ain't convinced by Fractal's diagnosis - I'd be spraying with an Ultimate Bug killer anyway, just in case.

i ment get rid of your leaves carefully as if say you drop one it could happen again etc .

i agree with bamboo to just in case.

I've disposed of leaves, well branches, in council wheelie bin. I have seen a few tiny, but long and narrow green creatures, fast moving, longer than aphid on some of the leaves, but could just be a coincidence. I'll try everyones remedy, belt and braces 'tho don't have any poisons...oh well...needs must:-( Thanks everyone for your interest and help, I'll probably end up with a branchless little tree lol, that'll teach it to attract erm..whatever it is:-)

your welcome .

Sorry, it is most probably this species Aceria pseudoplatani (Which attacks specifically the Sycamore, your tree is a form of it).

The fuzzy patches match much more closely, joining up like yours. They do range in colour from white to brownish and it is suspected that there may be more than one species that cause this gall, presumably the colour of the gall being significant.

What do they say? Mites may prove to be as successful biologically both in number of species and in the environments/habitats that they live in as Insects and might even equal or exceed them if they were ever all accounted for.

http://www.commanster.eu/commanster/Plants/Trees/Trees/Acer.pseudoplatanus.html

Thanks Fractal, as usual....you're right! It took me ages to find the page, had to go through Wiki. It shows aceria pseudoplatani, and it looks the same. I take it it's not a pest of Japanese acers?? I also presume there's no point trying to see the mite?? Does it have any natural predator??

And now I do agree with Fractal's diagnosis! Much more the ticket, though I'd never heard of either of these things till Fractal mentioned them.

We could pretend we had Bamboo, otherwise he may get big headed;-)

Good point, be awful if he couldn't get through normal size doors, lol!

:-)

Funny..... I can't lol

:-)))