Well, it's not Pelargonium rust, which is probably the most serious of
the fungal diseases that can attack them. Rust exhibits as pale
markings on the surface of the leaves and light brown or orange
pustules on the leaf undersides. It quickly spreads to all leaves and
causes yellowing followed by defoliation, which severely weakens the
plant. It used to be a major worry for commercial growers and I've
known nurseries destroy their entire stock when it has appeared. Some
varieties are far more prone to rust than others and it seems to be
far more damaging to regals.

The partial rings on the leaf undersides in the pic suggest one of the
leaf spot fungi, which are far less harmful and usually encouraged by
condensation remaining on the leaf undersides for too long. It can
happen when plants are overcrowded and/or when there's a combination
of warm days with cool nights and watering is carried out late in the
day. Invariably the infection disappears with the removal of the
affected leaves and I've never known it to be a serious problem.