Can athelete's foot fungus spread to your toenails as well?

Yes: The same fungi that cause athlete's foot can also cause onychomycosis: fungal nail infection. Unlike athlete's foot, it usually has to be treated with oral medications (if you choose to treat it).

Fungal toenails: Athletes foot can deftly spread to a patient's toenails if left untreated. The same type of fungus that cause athletes foot is the fungus that infects the nail plate.