Correct me if I'm wrong but corn used for Popcorn is not GMO

[Major Nikon]

133. Potatoes are tubers that grow underground

So an insecticide that is sprayed on the top of the plant is not going to touch the tuber. So while it may be possible for some of it to be absorbed through the plant and into the tuber, this would also be a concern for a tomato. Things like DDT, if they are present at all, would be just as likely in conventional vs organic because that product is no longer used for food production.

Chlorpropham is the anti-sprouting chemical used. It is non-carcinogenic and has an extremely low toxicity level which is about the same as table salt, yet the maximum dosage you could ever expect to receive from a single serving of potatoes is measured in micrograms and around 300 times lower than what the EPA's acceptable daily intake which is already many times lower than what can be expected to harm you in any way.

As far as pesticides go, you should have greater concern for those used in organic food production. For instance, you mentioned fungicide. One of the only effective organic fungicides available is copper sulfate. Now is copper sulfate really a concern as a pesticide residue on food? Not really. However, there's no question it's far more toxic and far less regulated than comparable products not approved for organic farming. It's also far less effective and far more persistent in the environment. This means farmers use more of it and it doesn't break down as readily.

It's important to remember that the term "organic" is nothing more than a marketing term with no regulations that address nutrition or safety.