The term “rhubarb” (meaning a heated dispute) is sometimes said to have started in 1938 or 1939, when a Brooklyn Dodgers fan shot and killed a New York Giants fan in an argument. New York sportswriter Tom Meany (of the World-Telegram and later PM) reported that a bartender called the incident a “rhubarb,” but the bartender’s name is not known and the meaning was not explained.
 
However, others credit the term to New York (NY) Journal-American sportswriter Garry Schumacher, who grew up in Greenpoint, Brooklyn. The Greenpoint (NY) Weekly Star on July 10, 1959 (see below) explained that Schumacher and other Greenpoint kids had rhubarb pie after school, and then got into fights and were often covered in rhubarb.
 
Brooklyn Dodgers radio sports commentator Red Barber (1908-1992) made frequent use of “rhubarb” and wrote two books with the word in the title—The Rhubarb Patch; The Story of the Modern Brooklyn Dodgers (1954) and Rhubarb in the Catbird Seat (1968). American humorist and author H. Allen Smith (1907-1976) titled two of his books Rhubarb (1946) and Son of Rhubarb (1967). The “rhubarb” term was always exclusive to baseball—and mostly New York City baseball—but is seldom used today.
 
   
Wikipedia: Red Barber
Walter Lanier “Red” Barber (February 17, 1908—October 22, 1992) was an American sports commentator.
 
Barber, nicknamed “The Ol’ Redhead”, was primarily identified with radio broadcasts of Major League Baseball, calling play-by-play across four decades with the Cincinnati Reds (1934—38), Brooklyn Dodgers (1939—1953), and New York Yankees (1954—1966). Like his fellow sports pioneer Mel Allen, Barber also gained a niche calling college and professional football in his primary market of New York City.
(...)
Barber became famous for his signature catchphrases, including these:
 
. “Rhubarb” – any kind of heated on-field dispute or altercation.
   
(Oxford English Dictionary)
rhubarb, n. and adj.
U.S. slang (orig. Baseball). A heated dispute, a row.
1941   N.Y. Times 19 May 20/6   There was what the boys call ‘a bit of a rhubarb’ in the eighth when Cavarretta tried to steal home… In the ensuing run-down, the Cubs charged Phil’s progress was illegally blocked by Lavagetto.
1943   Baseballing Jan. 369/3   A ‘rhubarb’, which has become Brooklynese for a heated verbal run-in, especially between players and umpires.
1949   Richmond (Va.) Times-Dispatch 17 Jan. 3/2   The citizen waiting for a streetcar yesterday was of several minds about the ‘rhubarb’ between the Virginia Transit Company and its drivers.
 
Brooklyn Newsstand
13 April 1940, Brooklyn (NY) Daily Eagle, “Dizzy Dean is Dodger Possibility” by Tommy Holmes, pg. 5, col. 4:
Probably Durocher’s sage advice kept the ex-cotton picker out of many a mess of rhubarb.
     
14 November 1942, Democrat and Chronicle (Rochester, NY), “Sports Eye View” by Elliot Cushing, pg. 16, col. 1:
(Tom—ed.) Meaney writes: ‘The first rhubarb of Branch Rickey’s administration in Montague Street already is a matter of history. 
 
Newspapers.com
23 August 1943, Fitchburg (MA) Sentinel, pg. 4, col. 2:
BASEBALL’S GARDEN TALK
Red Barber, who has become a sort of American institution by broadcasting the Dodgers’ ball games from the home field in Brooklyn, has coined a new word which has taken hold and promises to be a permanent part of the colorful language of sports.
 
He refers to any kind of verbal tiff on the playing field as “rhubarb.”
   
OCLC WorldCat record
Rhubarb
Author: H Allen Smith
Publisher: Garden City, N.Y. : Sun Dial Press, 1946.
Edition/Format:   Print book : Fiction : English
 
8 November 1946, The Sunday Courier and Press (Evansville, IN), “Notes from a Bookworm,” pg. 10B, col. 7 ad:
Another item that interested us was one involving “Rhubarb,” by H. Allen Smith. Red Barber, the baseball commentator, has been using a new interpretation of the word, which probably soon will become public property. To quote Mr. Barber, rhubarb refers to the disagreements and dickerings which occur between the players and the umpires while the game is in progress.
   
14 September 1947, St. Louis (MO) Post-Dispatch, “Ask Me Another” by Harold Tuthill, pg. 4C, col. 6:
“Recently I have seen the expression ‘rhubarb’ used in connection with arguments in baseball. How did it originate, where and when?”  Ralph S. Smith, Ste. Genevieve, Mo.
 
Tom Meany, the talented writer for PM in New York, said it happened in Brooklyn in July 1939. A tavern man stopped him and said:
 
“Did you hear about the rhubarb we had last night? Two Brooklyn fans were killed in a fight over the Dodgers.”
 
Meany told the story to Garry Schumacher, another New York baseball writer, who, in turn, related it to Red Barber, the sports broadcaster. Barber used it several times on the air and H. Allen Smith increased its popularity by calling one of his books by that name. Meany defined rhubarb as a discussion which had its inception in something that normally should not have happened.
 
OCLC WorldCat record
The rhubarb patch; the story of the modern Brooklyn Dodgers.
Author: Red Barber
Publisher: New York, Simon and Schuster, 1954.
Edition/Format:   Print book : English
       

10 July 1959, Greenpoint (NY) Weekly Star, “As I Find It” by Al Warren, pg. 5, col. 1:
THE FIRST RHUBARB
The word “rhubarb” is extremely common to sports fans. When an announcer or reporter is called upon to describe an argument on the field of play he immediately gets his point across with this reference.
 
The term originated right here in Greenpoint and old timers might recall when they themselves were involved in the “first rhubarbs.”
 
According to no less authorities than language expert Bergen Evans and author H. Allen Smith, the first man to use the term was Garry Schumacher, a former sports writer and current assistant to the president of the San Francisco Giants.
 
When Garry was growing up in Greenpoint he, and many other kids, had rhubarb pie waiting for them when they returned home from school each day.
 
Like all kids they were anxious to hit the streets as soon as possible so they often took the pie with them. Since rough-housing was the order of the day quite often they came back with rhubarb in their ears, hair and down their necks—this happened frequently enough and the squabbles became “rhubarbs.”
 
Garry coined the words as a sport writer but it was Red Barber who popularized it while broadcasting for the Brooklyn Dodgers before they made like young men and went West.
 
Old Fulton NY Post Cards
26 January 1965, New York (NY) World-Telegram and Sun, “Ask Daniel” by Dan Daniel, pg. 25, col. 4:
The designation “rhubarb” in baseball was used for the first time by Garry Schumacher, public relations man of the Giants, when he was a Brooklyn reporter. Rhubarb, when ripe, shows what looks like traces of blood when peeled.
 
OCLC WorldCat record
Son of Rhubarb
Author: H Allen Smith; Leo Hershfield
Publisher: New York : Trident Press, 1967.
Edition/Format:   Print book : English
 
OCLC WorldCat record
Rhubarb in the catbird seat
Author: Red Barber; Robert W Creamer
Publisher: Garden City, N.Y., Doubleday, 1968.
Edition/Format:   Print book : Biography : English
 
Google Books
The Dickson Baseball Dictionary (Third Edition)
By Paul Dickson
New York, NY: W. W. Norton & Company, Inc.
2009
Pg. 704:
rhubarb A ruckus with the umpire(s); a confused situation; a fight between players or between the players and fans; a stew; a noisy argument; a heated dispute. “Mr. ‘Red’ Barber has used the term ‘rhubarb to describe an argument, or mix-up, on the field of play (New York Herald Tribune, July 13, 1943; OED). Dizzy Dean’s definition adds: “Most of the fightin’ is done with their mouths.”
 
Google Books
Bush League Boys:
The Postwar Legends of Baseball in the American Southwest
By Toby Smith
Albuquerque, NM: University of New Mexico Press
2014
Pg. ?:
Sports Illustrated reported that in 1938 a Brooklyn Dodgers fan shot and killed a New York Giants fan in a barroom beef over the outcome of a ball game. The bartender, whose name no one remembered, described the incident to baseball writer Tom Meany as a “rhubarb.” What the barkeep meant by that is also lost. Radio broadcaster Red Barber heard the tale, liked the word, and began to say it frequently on his play-by-play of the Dodgers’ baseball games. Barber had a large audience and the word subsequently passed into the language. The popularity of “rhubarb” grew such that in 1951 the word became the movie title for a screwball comedy about a pet cat named Rhubarb who had been willed ownership of a baseball team.