I traced your question back to an interview that was done by author Michael Pollan back in 1998.  Michael is a college professor at UC Berkeley, and has written several books on healthy eating, but he does not have a science background.  This was during a time of great controversy over Monsanto's efforts to expand the GMO potato seed into new areas.  By contract, Monsanto owned the potato seed and the potatoes that were harvested from the planted fields. Michael interviewed a sixty year old Idaho potato grower, Dan Forsyth of Jerome Idaho.  According to the story, Dan grew potatoes as a business in a conventional manner. When asked about growing organic produce:“I like to eat organic food, and in fact I grow a lot of it at the house.  I’m not sure I should be saying this, but I always plant a small area of potatoes without any chemicals.  By the end of the season, my field potatoes are fine to eat, but any potatoes I pulled today are probably still full of systemics. I don’t eat them"(Page 220).You can read a lot into this and conclude as Michael has that conventionally grown potatoes, using fertilizers and pesticides, are automatically harmful to eat which organic potatoes use nothing and just grown fine without any help.

Here are some facts to consider:

1. All pesticides, fertilizers and fungicides used commercially are subject to very strict regulations by the EPA, even to the point that only certain ones are approved to be OK for a single crop and cannot be applied without following regulatory requirements set forth on the label.
2. If a crop encounters a problem with pests, rather than lose the whole harvest, a grower may use an integrated pest management program and that could include an application of a chemical.  It has to be done according to pre-approved guidelines and the necessary paperwork has to be filled out and kept on file for future reference.
3. However, the potatoes cannot be harvested or sold for human consumption until the crop meets minimum residue levels and traces of any chemical residual have dissipated. That's the law.
4. The growers comments were correct, "my fields are fine to eat"... but he should have said that he would have to wait to harvest the potatoes or would need to harvest and then store them till any residual trace of chemicals was below MRL’s.
5. So, he chose to also grow his own personal crop of vegetables organically which he can eat at any point in time, without having to wait.

To answer your original question, the Idaho potatoes you usually purchase at Costco are completely safe to eat (America has the safest food supply in the world with some of the strictest rules science can measure).