Luke 6:43 For a good tree bringeth not forth corrupt fruit; neither doth a corrupt tree bring forth good fruit. (par. Matt. 7:17)
Those who press this agricultural metaphor a little too far may say, "That's wrong! Just look at any tree with fruit on it, and you'll see that even the best tree makes rotten fruit now and then."
True, but that's not quite what Jesus has in mind. We're so focussed on the nice stuff we get in our produce department that we forget that there are worlds of fruit trees out there unfit for human consumption.
One of these, given as an exemplar in Malina and Rohrbaugh's social science commentary [323], is the Sodom's Apple, reputed in Jewish legend, according to Josephus, to have been one of the effects of the judgment on Sodom.
As these authors describe it, the tree is "low and unattractive" and while the fruit looks good enough to eat, its interior is full of seeds and tufts of hairlike growth that make it practically inedible and unpleasant to consume.
That's assuming of course that you get past the rind (which is "deadly poison") and manage not to be touched by the tree's sap ("a milky latex extremely irritating to the skin"). (A modern parallel might be drawn in my home state to the Tropical Soda Apple.)
Of course, it might be then argued that an apple tree might produce a rotten apple now and then; but if that's the fault of pests or drought, for example, that is not making the fruit inherently bad to us, by nature, which is the whole point of the parable (not that it can be MADE bad by outside effects). You may as well say that you could invalidate this teaching by going around to good fruit on the trees and scribbling profanity on it, and then smashing it to make it "bad".
-JPH