intreresting~! I don't know either with that~!

The noun suffix 子

Why can I add 子 to一个梨子or一个桃子，but why can't I add 子to 一个苹果

       Umm, your question is really interesting! 

       Well, as you said, we can say" 梨子", "桃子". This is because that in our spoken language, "梨子"、"桃子" is another way of saying "梨"、"桃", but we use it just in spoken language or a casual occasion.  In other words, these are the nicknames.  But "苹果"、 "香蕉”、"葡萄" and many stuff, they don't have their nicknames, so we can't add "子"

I've never thought about this question before. In my opinion, you could add "子" after a single-character-word to make it a bit longer, but it is not necessary to do that for a word like "苹果".

Of course, not every one-character-word can be added 子 in this way, for example, you CANNOT say "球子", "手子" or "床子". There is no universal rule for this, but you'll gradually learn it. :)