Kerria japonica 'Pleniflora' blight - Treatment Query
I I live in a wooded area at the highest point of South East London. I have an established row of Kerria japonica 'Pleniflora', planted 16 years ago, providing a high hedge for privacy. After usual superb flowering and good leaf growth, at the end of June this year, 2016, all 20 shrubs looked miserable, thin, covered with spots on leaves and some stems, several stems have gone dark brown and died, and 'red' leaves have dropped as if it was autumn.
I have treated the blight with a fungicide and fed the shrubs but the condition is not stopping, and it looks as if it is spreading to the Cotinus next to it.
I have been searching online to find out if this blight is anywhere else in the local area or in the UK but so far can only find a reference to an online report of 'Leaf Spot' on Kerria Japonica, caused by Corynespora cassiicola in Korea, in May 2015. The description matches the symptoms on my kerria but there was no solution.
I am looking for a diagnosis. How to treat the disease? And are there reports of this blight elsewhere in the UK?
Thank you for any information you can provide.