Is This A Leaf Spot?

This is the second yellowing leaf found within the past two weeks with a fungal attack at its base. And, as you can see (right), it has gone right through to the underside. Is it leaf spot? I don't know. Had a look through manuals but can't find it.

Although there was an infestation of leaf spot last year, started from an ivy leaf geranium brought in from a new source, it had a different appearance. This infection was walnut-brown with dark, concentric rings. So, if it is leaf spot, it looks like a different type.

All the plants which had been outside were treated with fungicide last year. But this plant was kept indoors, not exposed. There has never been anything like this before. Anyway, even if the plant were infected and if the fungus survived, why has it not appeared before now?

The yellowing leaf was taken from the over-watered plant recently described. So it might not be a leaf spot fungal infection; it might be a disorder caused by the over-watering, which has gone mouldy. Any thoughts?