One bad apple spoils the bunch

[snopes]

Comment: Here's an old one you don't cover on your site:

Does one apple _really_ spoil the whole bunch?

I know that putting apples inside a paper bag supposedly aids in ripening
(due to supposed the emission of some sort of chemical), so it's not
impossible that a rotten apple might somehow contaminate others near to it
(especially in an enclosed environment). But does it?

Is there any truth to the old saying? Will leaving a single rotten apple
with other apples actually cause the other apples to rot before their
time? Or is it just an urban legend?

[Spookshow Baby]

Well, if the apple starts sprouting mould it stands to reason that the other apples in the bunch would also become covered in mould spores and so rot faster.

[Horse Chestnut]

Orchardman's daughter here:

As Spookshow Baby stated above, an apple that is already infested with mold will contaminate the fruit around it, which in turn will contaminate the apples around them, until it spreads through the whole basket. That is why it is important to sort fruit and remove any blemished apples. One small cut can start the process of rot.

Also, ripening fruit give off a gaseous hormone called ethylene. The riper the fruit, the more ethylene is produced. When fruit is in storage, the presence of ethylene in heavy concentration can speed the ripening of the fruit. Overripe or rotten fruit gives off even more ethylene, which would cause all the fruit around it to ripen - and eventually rot - much quicker.

Here's a science project page that explains the process: Synchrony in Ripening Fruit.