On 04/06/2014 14:30, Davey wrote:
On Wed, 04 Jun 2014 13:09:22 +0100
Martin Brown wrote:

Is the white stuff woolly looking when dry?

I suppose so, but it's not how I would have described it myself. I
thought at first that it was sap leaking out and drying.

It could be either. Woolly aphid is a strong contender for the sort of
weeping wound with white fluff that you showed in your picture.

Basically wet some out with meths and if it is insect based you should
see the little blighters lurking in the cracks.

Sap in a willow is likely to be strong so it will come out. It is
unlikely to harm such a vigorous growing plant though. My parents have a
old apple tree that is a martyr to woolly aphid and I have never managed
to nuke it into submission it always reappears. Tree survives OK but it
would probably fruit better if the pest was cleared.

--
Regards,
Martin Brown