If it is confined to just a few plants then destroy those affected by burning them or putting them in your household waste bin do not add them to the compost heap! The remaining plants can be sprayed on a dry day with a fungicide that is suitable for use on ornamentals.
Many fungal problems can be reduced by good hygienic cultivation practices. Water the plants from beneath the foliage rather than over the top, as this may cause the plant to rot and also creates ideal conditions for the spread of fungal spores. Don’t crowd the plants too closely together - the better the ventilation, the less the risk of fungal infection. Also, remove and destroy any infected plant material as soon as you spot it.
Finally, use fresh, sterilised compost when potting up plugs and before reusing pots then they should be thoroughly washed clean.