Hi Dodger,

This can be caused by three different types of infectious agent - bacteria, fungus or virus. The first two are readily treatable with copper-based fungicides (these are reasonably effective against bacterial infection too) if used regularly enough, whilst a viral infection is not treatable.

Given the recent period of drought then bacterial infection due to stress is the most likely possibility but if you could supply a pic of the damage we could try to narrow it down. If the staining is confined to the flowers only then it is not likely to be hugely damaging, but if it's present on the foliage too then the problem is more serious.

Thanks very much Ilex. The staining is confined to the flower. These are potted plants. Could you recommend any particular fungicide.
Once again many thanks.

No problem Dodger, it sounds like your problem is not too damaging to the plants, although it's a shame that the flowers are being spoiled of course.

Bordeaux Mixture is the traditional and widely available copper-based fungicide, but there are all sorts of other rival types that should be available from any garden centre. It's also worth making sure the plants are kept well watered as this will help them fight off infection more easily.

Will do, and thanks again.