Back when I was in conservatory, I heard lots of students and faculty members dismiss George Gershwin (1897-1938) on the grounds that, “He couldn’t even orchestrate his own compositions.”
I was reminded of that opinion the other day when I was listening on my car radio to “Catfish Row,” an orchestral suite from Gershwin’s opera Porgy and Bess. Boy, are those melodies wonderful. Boy, is that orchestration good. But I was unsettled to realize that the orchestration of that work simply sounds a lot different from that of An American in Paris,which has been getting a lot of air time recently because of an upcoming Broadway show based on it. And both those orchestrations sound different from the orchestration of Rhapsody in Blue.
That led to wonder whether different people had orchestrated those works. So I did a little research and the answer is, apparently so.
Rhapsody in Blue (1924)
Ferdy Grofé, a master orchestrator and a fine composer in his own right, orchestrated Rhapsody in Blue prior to its first performance by the Paul Whiteman orchestra on February 12, 1924, with Gershwin as piano soloist.  Grofé was the orchestrator for the Paul Whiteman orchestra at the time and there seems to be copious documentation that he orchestrated the piece and wrote out the parts for the premiere.
In case you are thinking how sad it is that that famous ascending clarinet swoop that starts the piece was not actually written by Gershwin, apparently the truth is a bit more complex. The clarinet player for the Whiteman orchestra reportedly played his part that way in a rehearsal and Gershwin liked it so much that it has been played that way ever since. So that clarinet wail is apparently the result of collaboration. Over the years, various conductors have apparently tinkered with the orchestral parts too. So the orchestration of the piece is apparently in flux.

Here’s a recording of Gershwin himself playing his piece with the Whiteman orchestra. The way the band’s instrumentalists approach their solos offers an aural time capsule of the way that Gershwin heard this piece. It sounds a lot different from what we hear in orchestral concerts today.

Porgy and Bess (1934)
George Gershwin apparently orchestrated this opera prior to its premier in 1934, after studying composition and orchestration with Josef Schillinger, who also taught Benny Goodman and Tommy Dorsey. Over the years, some Gershwin critics have claimed that Schillinger actually orchestrated the opera; apparently that is not so.
But what about that orchestral suite from Porgy that I heard on the radio? It turns out that what I head was “Catfish Row,” a suite that Robert Russell arranged in 1942, based on themes from Porgy. Though based on Gershwin’s scoring, it features a lot more solos for various instruments – including a banjo in “I Got Plenty of Nothin’.” 
An American in Paris (1928)
Gershwin apparently completed the original orchestration for this work just prior to its first performance in 1928. What gets the mind and ear confused, however, is the American in Paris score that has become most familiar – that is to say, the music that we hear in the 1951 film starring Gene Kelly. That version was arranged and orchestrated by Johnny Green, and it is considerably lusher than Gershwin’s score.
It All Gets Confusing, Right?
Yes, it does. But whatever the realities may be about who did the first of Gershwin’s orchestrations and who revised them, I think we have to accept the idea that he is one of our most important American composers. Anybody who wrote melodies like “Summertime” or “Bess, You Is My Woman Now” and the others cannot be marginalized. Anybody who created the unique mood of An American in Paris was a genius, no matter who fleshed out his orchestral parts.
Then I come to the fact that people who have denigrated Gershwin as a poor composer because he was weak orchestrator might not be applying similar criticism at “serious” classical composers. Does anybody dismiss Mussorgsky because we only hear his piano work Pictures at an Exhibition in orchestrations by either Rimsky-Korsakov or Ravel? Does anybody say that Schumann or Beethoven are second-class composers because conductors have made “corrections” in their orchestrations over the years? And let’s face the fact that the orchestrations that Chopin provided for his two piano concertos are a bit ragged, despite various editings and “improvements” over the years.
I am all for purism. But maybe we would all be better off if we just sat back, listened, and perhaps even enjoyed music after it has traveled its complex path from the composer’s mind to our ears.