So every good tree bears good fruit, but the bad tree bears bad fruit: houtos pan dendron agathon karpous kalous poiei, (3SPAI) to de sapron dendron karpous ponerous poiei; (3SPAI): (Every good - Ps 1:3; 92:13,14; Is 5:3, 4, 5; 61:3; Je 11:19; 17:8; Lk 13:6, 7, 8, 9; Gal 5:22, 23, 24; Ep 5:9; Php 1:11; Col 1:10; Jas 3:17,18) (But -  Mt 12:33, 34, 35; Jude 1:12)

Though Jesus moves from bushes to trees, His basic point is similar. In this passage Jesus is concerned not with the kind of fruit (as in the former passage) but with the quality of the fruit as well as the quality of the tree.

Wiersbe notes that the two trees...

These show that true faith in Christ changes the life and produces fruit for God’s glory. Everything in nature reproduces after its kind, and this is also true in the spiritual realm. The second test (of one's profession of faith as to its genuineness) is this: Did my decision for Christ change my life?...The person who believes false doctrine, or who follows a false prophet, will never experience a changed life. Unfortunately, some people do not realize this until it is too late.  (Wiersbe, W: Bible Exposition Commentary. 1989. Victor)

Good (tree) (18) (agathos) describes that which is "good" in its character or constitution, beneficial in its effect, profitable, useful, good (value), pertaining to different objects, ie., soil (Lk 8:8) having the proper characteristics or performing the expected function in a fully satisfactory way.

Good (fruit) (2570) (kalos) does not refer to that which is superficial or cosmetic but to what is genuinely and inherently good, righteous, noble, and excellent. In classical usage,

Kalos originally as descriptive of outward form, beautiful; of usefulness, as a fair haven, a fair wind. Auspicious, as sacrifices. Morally beautiful, noble. 

Kalos means outwardly fair, as the stones of the temple (Lk 21:5): well adapted to its purpose, as salt (Mk 9:50): competent for an office, as deacons (1Ti 4:6); a steward (1Pe 4:10-note); a soldier (2Ti 2:3-note): expedient, wholesome (Mk 9:43, 45, 47): morally good, noble, as works (Mt 5:16-note); conscience (Heb 13:18-note). In the Septuagint kalos is the most usual word for good as opposed to evil (Ge 2:17; 24:50; Isa 5:20).

Gill observes that...

As is the tree, so is its fruit; if the tree is good, it will bring forth good fruit. The tree that brings forth good fruit, is good antecedent to the fruit it produces; it is first good, and then puts forth good fruit: it is not the fruit that makes the tree good, but makes it appear to be so; but it is the goodness of the tree that makes the fruit good.

As a good man does, and will do good works, but his works do not make him a good man; he is so before he performs good works, or he would never be able to do them; these make him appear to be a good man: so a good preacher, that has an experimental knowledge of the doctrines of the Gospel, will deliver out sound doctrine, who is first made so by the gifts and graces of the Spirit of God; and by searching the Scriptures, and examining his doctrines by them, he will be known and appear to be a good minister of Jesus Christ, nourished up in the words of faith and good doctrine; and such a good minister of the Gospel, out of the good treasure of Gospel truths put into his earthen vessel, will bring forth, from time to time, good and excellent truths, to the edification and profit of those that hear: "but a corrupt tree bringeth forth evil fruit"; if the tree is corrupt, the fruit will be corrupt; and as is the preacher, so will be his doctrines: if he is a corrupt preacher, or a man of a corrupt mind, destitute of the truth, his preaching will be such as will tend to corrupt both the principles and practices of men; for such evil men and seducers, out of the evil treasure of false doctrines, which they have received into their judgments, will bring forth, either more secretly or openly, evil tenets in their ministry, which prove of bad consequence to the souls of men (Ref)

Adam Clarke reminds us...

that as the good tree means a good heart, and the good fruit, a holy life, and that every heart is naturally vicious; so there is none but God who can pluck up the vicious tree, create a good heart, plant, cultivate, water, and make it continually fruitful in righteousness and true holiness.

Bad (tree) (4550) (sapros from sepo = cause to decay, to putrefy, to rot away, be corrupted) means rotten, putrefying, corrupt, disgusting, perishing. Sapros described spoiled fish, rotten grapes on the ground, crumbling stones. The basic meaning relates to the process of decay. Obviously Jesus is not using sapros literally, for if the tree were totally decayed it could yield no fruit at all. His point is that the tree is far from healthy and this is shown by its inability to bear good fruit.

Bad (fruit) (4190) (poneros) refers to evil in active opposition to good.

Regarding the fact that a "tree is known by its fruits" Barnhouse wrote that...

This is an infallible criterion, both in the natural and spiritual realm. ‘Even so every good tree bringeth forth good fruit; but a corrupt tree bringeth forth evil fruit’ (Matt. 7:17). The test for love of Christ is obedience to His commandment. Profession is proved or disproved by the daily walk. If a man has no love for spiritual things, he is devoid of a spiritual nature. If a man is in an utterly prayerless state, he has not received the spirit of adoption whereby the saved cries ‘Abba, Father.’ If a man is thoroughly wrapped up in the things of this world, then his eyes must be closed to the glories of Heaven, ‘For where your treasure is, there will your heart be also’ (Mt 6:21-note). If a man prefers the company of worldlings to that of God’s people, then he is a worldling himself. If a man lives to please self rather than God, he is yet dead in trespasses and sin. (Barnhouse, D. G. God's Freedom: Romans 6:1-7:25 page 247. Grand Rapids, MI.: William B. Eerdmans Publishing Company)

John Blanchard put it this way...

A fruitless person is not a failed Christian, but a false one—in other words, not a Christian at all...Fruit is evidence of the root. (The Complete Gathered Gold)

Bad fruit - A New York City couple were mailed two tickets to a smash Broadway hit with no explanation or identification of the sender. Nevertheless they decided to attend the show which they thoroughly enjoyed. Returning after the show that night, they discovered their home had been ransacked and looted of furs and jewelry. On the pillow was this simple note: "Now you know." Like that nameless thief, false prophets and false teachers know what people want to hear and so they proclaim a message that appeals to sensual desires (see notes beginning 2 Peter 2, especially 2Pe 2:2-note). These spiritual charlatans don't wear "warning labels", but are servants of Satan who "disguise themselves as servants of righteousness" (2Co 11:15). They are masterful spiritual chameleons who make stupendous claims, but in due time their followers usually end up paying a high price, which can even be eternal destruction if they never hear the truth, are granted repentance, come to their senses and escape from the snare of the devil's servants, having been held captive by them to do their will (2Ti 2:24, 25, 26-notes). Even believers can be deceived by false prophets and false teachers. That is why God's Word repeatedly exhorts us to study the Scriptures (1Pe 2:2-note), test what we hear (1Jn 4:1, Acts 17:11-note), and grow in the faith (2Pe 1:5, 6, 7, 8, 9 - see notes 2Pe 1:5,  6-7, 8-9). That way, we won't someday go to our "home" and find out that it was not the home in which we expected to spend eternity (Mt 7:21-note)

Beloved, do not believe (stop believing = present imperative with a negative - indicating they were already falling prey) every spirit, but test (present imperative - carry out a careful examination to prove a thing worthy or genuine) the spirits to see whether they are from God; because many (not just a few!) false prophets have gone out into the world. (1John 4:1)

As an aside John Stott makes the pithy comment that...

A good tree cannot produce bad fruit, nor can a bad tree produce good fruit: ou dunatai (3SPPI) dendron agathon karpous ponerous poiein, (PAN) oude dendron sapron karpous kalous poiein. (PAN)  (Galatians 5:17; 1 John 3:9,10)

Spurgeon writes...

After all, this is the best test of any doctrine, the practice to which it leads. I remember one day discussing with a person about the doctrine of future punishment. We were arguing, and the gentleman, who owned the vessel on which we were, said, “Come up on deck, and enjoy the fresh air, and leave that subject; but,” he said, “you, sir, will kindly go as far as possible from my men, for they are bad enough as they are, and if you tell them there is no punishment for sin, they will be worse than ever. As for you, Mr. Spurgeon, you may go where you like, you won’t do them any harm.” I thought that rough and ready mode of argument was about as good a commendation as I could wish to have.

Good (tree) (18) (agathos) describes that which is "good" in its character or constitution, beneficial in its effect, profitable, useful, good (value), pertaining to different objects, ie., soil (Lk 8:8) having the proper characteristics or performing the expected function in a fully satisfactory way.

Good tree as Wuest renders it is the "intrinsically good tree" which is good because it is planted by God (Mt 15:13 "Every plant which My heavenly Father did not plant shall be rooted up." ), belongs to Him and bears good fruit, namely it does His will.

Cannot produce bad fruit - Absolutely cannot do this. And vice versa. Jesus' point is that it is impossible for a tree to bear fruit which is contrary to its nature. Healthy trees cannot bear the kind of fruit that is natural to the bad tree. Conversely, the bad tree completely lacks the ability to bear good fruit.

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Counterfeit Reality - When people see a photograph or video today, they often ask, "Is it real?" A home computer can manipulate images to create a picture of an event that never happened. Images can be inserted into or removed from photographs. A video can be doctored to make it appear that a person was caught committing a crime or performing an act of heroism. The camera may not lie, but the computer can.

Centuries before such modern technology, the apostle Paul warned Timothy about counterfeit reality in the church. He said that in the last days people would be self-absorbed, "having a form of godliness but denying its power" (2Ti 3:5-note). He repeatedly emphasized the need to live a godly life, warning that "evil men and impostors will grow worse and worse, deceiving and being deceived" (2Ti 3:13-note).

Paul charged Timothy to "continue in the things which you have learned and been assured of" (2Ti 3:14-note). True godliness honors and obeys God while its counterfeit seeks pleasure and personal gain. One pleases the Lord; the other gratifies natural desire. Both are identified by their actions.

When people hear us say we are Christians, they may wonder if our faith is real. Our lives will answer the question by reflecting the reality of Christ. —David C. McCasland  (Our Daily Bread, Copyright RBC Ministries, Grand Rapids, MI. Reprinted by permission. All rights reserved)

Every tree that does not bear good fruit is cut down and thrown into the fire: pan dendron me poioun (PAPNSN) karpon kalon ekkoptetai (3SPPI) kai eis pur balletai. (3SPPI): (Mt 3:10; 21:19,20; Isaiah 5:5, 6, 7; 27:11; Ezek 15:2, 3, 4, 5, 6, 7; Luke 3:9; 13:6, 7, 8, 9; John 15:2, 3, 4, 5, 6; He 6:8; Jude 1:12)

FROM
THE FRUIT
TO
THE FATE

Jesus moves from the fruit to the fate of the rotten tree, giving a judgment similar that of John the Baptist who declared...

And the axe is already laid (= time is running out; judgment is near) at the root of the trees; every tree therefore that does not bear good fruit is cut down and thrown into the fire. (Mt 3:10)

Every tree - All without exception. No bad, rotten or worthless "tree" (false prophet) is allowed to continue producing bad fruit.

Cut down (1581) (ekkopto from ek = out + kópto = cut) which literally means to cut or strike out and so to cut down. Here Jesus uses it to describe the trees (false prophets) who are done away with or eliminated, as in the tragic example below.

Ekkopto is used 10 times in the NT - Matt. 3:10; 5:30; 7:19; 18:8; Lk. 3:9; 13:7, 9; Ro 11:22, 24; 2 Co. 11:12.

There are 37 uses of ekkopto in the Septuagint (LXX)- Gen. 32:8; 36:35; Exod. 21:27; 34:13; Num. 16:14; Deut. 7:5; 12:3; 20:19f; Jos. 15:16; Jdg. 16:21; 21:6; 1 Ki. 15:13; 2 Chr. 14:3, 14f; 31:1; Job 14:7; 19:10; 42:17; Ps. 74:5; Prov. 30:17; Isa. 9:10; 27:9; Jer. 6:6; 10:3; 22:7; 44:7f; 46:23; Dan. 2:40; 4:14, 17, 23; 9:26; Mic. 5:14; Zech. 12:11
 

Adam Clarke comments...

What a terrible sentence is this against Christless pastors, and Christless hearers! Every tree that produces no good fruit is to be now cut down; the act of excision is now taking place: the curse of the Lord is even now on the head and the heart of every false teacher, and impenitent hearer.

Do you remember the name Jim Jones? He was the head of the People's Temple Christian Church that began in California and ended at Jonestown in the jungles of Guyana, South America when he and nearly a thousand of his followers committed suicide. The amazing thing is that most of those who joined Jones' church were from Christian backgrounds!

MacArthur quotes Mel White's book "Deceived" in which he analyzes why so many were so misled and suggests that Jones...

"knew how to inspire hope. He was committed to people in need; he counseled prisoners and juvenile delinquents. He started a job placement center; he opened rest homes and homes for the retarded; he had a health clinic; he organized a vocational training center; he provided free legal aid; he founded a community center; he preached about God. He even claimed to cast out demons, do miracles and heal. (see notes Matthew 7:22). But on the other hand we find all the marks of a false prophet. He promoted himself through the use of celebrities, a very common vehicle for false prophets to gain credibility. He manipulated the press; he wanted certain favorable stories; he was big on playing the press... and he used the language and the forms of faith to gain his power. (Bolding added)

J C Philpot comments on trees that don't bear fruit writing that...