It is usually not a serious problem. It may cause some reduced growth. Yes, especially if temperatures remain between 55-65 F and conditions are wet. The disease is called scurf and it is caused by the fungus, Monilochaetes infuscans. The damage is only skin deep and it does not affect edibility of the roots.
Symptoms are typically leaf spots and blotches.
Blotches often occur along the side of leaf veins. Often the problem involves root damage. In the winter, geraniums may develop yellow spots on upper leaf surfaces with small brown corky, slightly raised spots on corresponding lower leaf surface areas. The condition is called oedema. It is caused usually after several cloudy days when roots absorb water normal for sunny days.