Any clues as
to the cause? and more importanly a suggested treatment? or should I give
up and try again with new stock?

Nothing else in the bed is affected.

Thanks
--
William Tasso - http://williamtasso.com

Cause of many infestations on plants is imbalance/deficiency of something
within the plant itself. The suggested remedy for rust infestation is the use
of a high potassium fertiliser (which probably means that a deficiency of
potassium within plants is a contributing factor with regard to rust). Remove
the leaves that are affected. Spray with a systemic fungicide.

Sometimes when I think a plant is possibly too 'ill' to make a recovery, I take
a cutting, or do something similar to ensure I don't actually 'lose' the plant
all together, though I may actually lose the 'original' plant. I hope that
makes sense. I'd suggest removing the odd stem from each of your Cornus and
removing all leaves, wash in a detergent, then place in a clean container
filled with a reasonable amount of water. Cornus like lots of water and often
root when left in water, even when you don't want them too.

Hope this helps.

Dave.