We don't know what caused the tan spots to develop on the leaves.  Fungal or bacterial leaf spots appear on many kinds of garden plants.  Several kinds of fungus called "botrytis" account for these but we can't be sure that's what's causing the spots on the plant in the photo. Insects might also be responsible but we don't see any evidence of them. 

Dampness promotes many fungal diseases so keeping the leaves as dry as possible, especially when watering may help. 

Despite the spots, the plant in the photo appears to be healthy.  Unless the damage spreads rapidly and the plants decline, there may be nothing to worry about.  If the condition of leaves and stems deteriorates significantly, try cutting them back.  It may take the plants two or three weeks to recover but such pruning often results in a flush of new growth and flowers in late summer/fall.