Jesus" point in this parable was that a person of bad character cannot normally produce good conduct (cf. Matthew 12:33-35). Therefore His disciples needed to clean up their lives before they could minister for Him effectively. As a pupil follows the example of his teacher ( Luke 6:40), so fruit from a tree follows the nature of that tree. In the Matthew parallel Jesus applied the parable to false teachers, but here it stands by itself and applies in this context to disciples of His. Conduct follows character as surely as fruit follows root, for good and for bad (cf. James 3:12). The conduct of Christians is sometimes bad rather than good because our character is still sinful. We are not totally good or totally bad. [source][source][source]
"The text indicates that although fruit may not be a certain indicator, it can be a suggestive one." [1][source]