Blackspot
How to recognise it
Dark, roughly circular spots on rose leaves, often with a yellowing halo, leading to premature leaf drop.
Life cycle and conditions
Spores spread by rain splash from fallen leaves and overwintering material; worse in wet summers and on crowded, poorly ventilated plants.
Prevention
Clear fallen leaves promptly (don't compost them), avoid wetting foliage when watering, improve airflow through pruning.
How to treat it
Remove affected leaves as they appear and keep up the hygiene routine in prevention — resistant rose varieties reduce the problem more effectively than any spray, if you're choosing new plants.
A rose-specific fungicide is available for amateur use as a last-resort preventative on badly affected, valued plants — fewer products remain approved than in the past, so good hygiene does most of the work regardless.