Mildew on blackberries on bush

My thornless blackberry bush has been a great producer of fruit and this year looked set to be a bumper year. However, when I go to pick the fruit I have to discard the majority of berries as they seem to be affected by some blight that looks like mildew. Has anyone any ideas as to what might be the cause of this problem? Only the fruit is being affected and not the rest of the bush.