Where do potato tubers "come from?"

When we grow potatoes from tubers bought at nurseries, etc, in the early spring, the tubers must be specifically cultivated for this purpose?

Where do the tubers come from? Are they the leftovers from the previous year's crop? Or were they grown specifically to kickstart the year's potatoes/

Are they grown in special conditions, or harvested early?

 

"Or were they grown specifically to kickstart the year's potatoes"

Yup, and in particular are certified virus and disease free.

 

I also read that grops above a certain altitude are more reliable free from pests and deseases

 

I worked on a plant nursery in Japan growing virus-free plants by tissue culture - that was at 4,500 feet to reduce the chance of infection from aphid / insects.

Great spot, on the edge of a cliff with a view down the valley to Mount Fuji 80 miles in the distance ...