What diseases affect geraniums?
Some of my geraniums have scale like spots on the underside of the leaves. The leaves turn yellow.
Your plants could have a pest issue or Virticillium can attack geraniums.
In the case of verticillium the plant would need to be disposed of.
Likely the plant just needs some watering adjustment and a Neem Oil treatment generally takes care of most insects or fungus.
Here are some links for you.