brown / black spots on heuchera
Please somebody help! About a month ago I planted a lovely healthy heuchera 'lime marmalade', at the front of a shady flowerbed where it is in sun for about 3 hours a day but shade the rest of the time. It has now developed blackish spots all over it, about half a centimetre across on almost every leaf. A hunt through google suggested that this was 'leaf spot' and that sterile pruning was the answer - but if I cut off every affected leaf there won't be any left. It has set in incredibly quickly, the plant was fine a week ago. Does anyone have any advice? Should I try pruning? or just pull it up and dispose of it before it affects my other heucheras?
To make it more complicated, I garden organically so any fungicide would need to be organic.
Thank you for any help!