[jonewer]

I bought a Forsythia a week or two ago from pound stretcher. It was a bare-root job and already in leaf.

I soaked the roots and planted and it seemed to be doing well until today when I noticed the leaves are severly mottled with a blanched looking affliction. 

Does anyone know what is causing this and what to do about it?

Thanks!

[Trillium]

Problem is that the plant was not first hardened off before going outside. It went from the weak light of a shop and straight into full spectrum daylight - hence, the leaf blanching.

You could leave it outside and hope new healthier leaves appear. Or, you can dig up the plant, pot it, leave it inside but daily put it outside for 30 minutes or so to slowly adapt to the much stronger outside light. In a week you can leave it out an hour then build up to a few hours. In 3-4 weeks you can replant outside. You'll not get flowers this year since it's already leafed, but it will perform next year for you. Don't feed with high nitrogen fertilizer as this will overly stress the plant.

[jonewer]

Thanks Trillium, thought that might be it but was hard to imagine anything getting sunburnt in this watery sunshine.

I'l dig it up and repot. (un)fortunately I have a north east facing garden so I'll place it somewhere in the shade then move it to full sun over the course of the next month or so.

[Trillium]

Even watery sunshine contains UV rays, which are doing the damage. I've gotten sunburned on enough overcast days to know it can and does happen.