In Things the Bible Doesn’t Say (But You Thought It Did), Notre Dame professor Candida Moss wrote, "Most people grow up learning that Eve took an apple from the Tree of Knowledge of Good and Evil, gave it to Adam, leading to the ejection of humanity from the Garden of Eden. But there’s no apple in the Garden—there’s only a piece of fruit. The interpretation that it’s an apple sneaks in in the King James Version of the Bible."
Moss may have a Ph.D. in Religious Studies from Yale, but I'm not too sure about her English comprehension. I've been reading the King James since I've been able to read the Bible, and never heard tell of such. Just to be sure, I ran several searches for the word "apple" in the KJV. I found "apple" or "apples" in 11 instances in the King James Bible -- none of which have any reference to Eve or Adam in the garden of Eden. There is no mention of the fruit that Adam and Eve ate being an apple found in the book of Genesis or any other place in the Bible (in any Bible versions I've examined). I wrote to ask her where she found this, but Moss didn't deign to answer. So...
After spending a good deal of time twiddling around online, I found something at Biblical Hermeneutics Stack Exchange ("question and answer site for professors, theologians, and those interested in exegetical analysis of biblical texts"). According to a comment there, the Interpreter's Dictionary of the Bible refers to "an early mistaken identification of the tree of Song of S. 8:5 with the story." I don't have Interpreter's Dictionary of the Bible, so I can't verify the statement or know whether it pointed to a specific instance (it was not quoted as if it did). But, curiously, I could not find any commentaries that make such an identification of the fruit. At any rate, Moss is wrong that the idea that the "forbidden fruit" as "an apple sneaks in in the King James Version of the Bible." It may have "snuck in" to someone's opinion about the Bible, but it is not in the Bible itself.
I did find 3 interesting suggestions for the source of the tradition among some Christians that the Tree of Knowledge of Good and Evil was an apple tree.
The Latin language and a possible pun
The Latin word for evil is malum and the Latin word for apple is also malum. The words mali/malum are found in the Vulgate in Genesis 2 and 3 references to "good and evil" (boni et mali/bonum et malum). Compare to Song of Solomon 2:3 (sicut malum inter ligna silvarum). [Vulgate] If this is true, we would not expect to find any "early" (i.e. Hebrew) examples of this tradition regarding the apple, but post-Vulgate. In the Hebrew language the words for "evil" and "apple" are not closely related.
The generic meaning of apple in Old English
According to the Online Etymology Dictionary, "the Old English æppel apple [means] any kind of fruit; fruit in general" and in "Middle English and as late as 17c., it was a generic term for all fruit other than berries but including nuts..." (Online Etymology Dictionary, © 2010 Douglas Harper). It would then have been natural for English speakers of this period to refer to the "apple" generically in writing or speaking of the fruit in Genesis 2 and 3, and this could have been passed down. If the association began here, we we would not expect to find examples of the idea in other languages, but simply in English.
The use of "apple" by John Milton
The first edition of John Milton's epic poem "Paradise Lost" was published in 1667. It is about the biblical story of the fall of man, and refers to the "apple" in a couple of places.
For example, in 10.482-493:
Long had foretold, a fabric wonderful
Of absolute perfection. Therein man
Placed in a Paradise, by our exile
Made happy: Him by fraud I have seduced
From his Creator; and, the more to increase
Your wonder, with an apple; he, thereat
Offended, worth your laughter, hath given up
Both his beloved Man, and all his world,
To Sin and Death a prey, and so to us,
Without our hazard, labor, or alarm;
To range in, and to dwell, and over man
To rule, as over all he should have ruled.
Milton's poem could have been responsible for popularizing the connection of the apple with the fruit of the Tree of Knowledge of Good and Evil. Even so, in 1667 he may have been using it in the more generic sense as opposed to actually meaning the apple as we know it -- Malus domestica.
In fine, if one means "apple" in an Old English sense of fruit generically then that is correct, but likely will be misunderstood. Most moderns think of something much more specific when they read or hear the word "apple". The Tree of the Knowledge of Good and Evil had real edible fruit, but it was not an apple (Malus domestica). Each tree God created bore fruit after its own kind (Genesis 1:12) and the Tree of the Knowledge of Good and Evil had its own fruit.