The rules for incorporeal creatures states that "Even when hit by spells or magic weapons, [an incorporeal creature] takes only half damage from a corporeal source (except for channel energy)." If, as a corporeal caster, I cast fungal infestation at an incorporeal creature, does it suffer the full effects of the ability damage, or does it only take half?
In the event that you do roll to hit and pass the 50% chance stated in the description for incorporeal creatures when attacking with spells that don't do damage, no. The damage that it mentions is hit point damage from corporeal sources. The ability damage would not be halved. However, you should take the following into account:
An incorporeal creature moves silently and cannot be heard with Perception checks if it doesn’t wish to be. It has no Strength score, so its Dexterity modifier applies to its melee attacks, ranged attacks, and CMB.
While your spell would take full effect so long as it was able to hit the creature and wasn't a damage spell, incorporeal creatures don't have a strength score, so you would be subtracting something from nothing, in this instance.
But the answer is Yes. Your spell would take full effect as long as it wasn't doing damage of the hit point variety. The Touch of Idiocy spell does stipulate that your spell must target a living creature, however, so ghosts are out, as well as any other undead.
Damage is defined on the d20pfsrd as the following:
If your attack succeeds, you deal damage. The type of weapon used determines the amount of damage you deal.
Damage reduces a target's current hit points.
Ability damage is similar, but it denotes a temporary loss of ability scores, the loss durates when the spell ends, and isn't defined under the damage header. It shares an aspect with Ability Drain, which is a permanent loss.