Looking at the photo, I think it could be blight that has been affecting Kerrias. Have you noticed leaves dropping off pre-maturely? Take a look at the link to see. There is also advice.
Looks like blight. I've just had this appear on my kerria and it has to go. It grows through the hedge so will be difficult to control but I have no intention of spraying fungicide all over the place on the offchance it might help.
I'm so gutted it flowered so well every spring and looked lovely for weeks, l was going to just cut it back but decided to dig it up and find something to take its place.
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