What Causes My Potatoes to Get Knobby or Deformed?
Store potatoes and my seed potatoes are nicely rounded but my homegrown potatoes get knobby and deformed making them hard to peel. Am I watering too much?
This is typically caused by uneven watering and is more likely to happen during times of drought.
This link has a lot of information that may be helpful to you:
https://www.extension.iastate.edu/publications/pm1890.pdf (pg 2)