Which doctor treats onychomycosis and /or fungal nail diseases in general?

Dermatologist. Onychomycosis hands and feet could be treated by a dermatologist, if toe nails are your only problem then you could be treated by your podiatrist.
Foot and Ankle/Derm. You should go see your local foot and ankle specialist (podiatrist/foot and ankle orthopedics), dermatologist or primary care physician.
Primary care. Most cases of nail fungus can and should be treated by one's primary care doctor. A simple scraping or culture will help to confirm the diagnosis. However, some types of nail fungus can signal a underlying immunodeficiency. So your doctor should help guide your care and possible referrals.