I don't know if this is an old wives tale because a Google search didn't bring me conclusive proof: in fact the top two results have exactly opposite answers on this question. Apparently both apples and onions contain ethylene gas but storing onions with potatoes is a big no no whereas I've been recommended several times to throw an apple into my bag of potatoes.
Does storing an apple with potatoes prevent the potatoes from sprouting
appleschemistrypotatoesstorage-lifetimestorage-method
Best Answer
According to this article, the advice to not store onions and potatoes together has nothing to do with ethylene. Onions do not give off ethylene. It has to do with moisture. Both onions and potatoes give off some moisture. Storing them together makes both more likely to rot.
Apples, on the other hand, do give off ethylene. This begs the question, "What does ethylene gas do to potatoes in storage?" Here's what I found.
Summary:
If your potatoes are in the endodormant stage ethylene will break them out of endodormancy and move them into the ecodormant stage. Potatoes in the endodormant stage are very shelf-stable. Potatoes in the ecodormant stage are less shelf stable.
Once your potatoes are already in the ecodormant stage, there's no disadvantage to supplying them with ethylene. Ethylene will inhibit sprout growth at this stage.
But, you can't tell if your potatoes are in the endodormant stage or the ecodormant stage. The only way you can tell what dormancy stage they're in, is when they start to sprout. Then you know they're not dormant anymore.
So, you want to either:
Once you have supplied your potatoes with ethylene, don't take away the source of ethylene.
Conclusion:
Don't:
Do:
-Or-
Both options require you to check on your potatoes occasionally. If you don't want to do this (or don't trust yourself to remember), don't store them with an apple. Better to risk a bit of sprouting than the rot that a rotting apple will cause.