Indeed, Rajahs are in India but from the notes below from Mainly Norfolk, it appears that anything vaguely eastern used to get attributed to Turkey.

Turkey Rhubarb
[ Roud 1073 ; trad.]

Tim Hart and Maddy Prior sang Turkey Rhubarb in 1969 on their second duo album Folk Songs of Old England Vol. 2. Their record's sleeve notes comment:

Another Bedfordshire song from the collection of Fred Hamer this one being from the singing of Harry Scott. Fred describes this as a “cross between a signature tune and a street call used by a hawker who carried rhubarb in season as part of his stock-in-trade”. Although rhubarb in fact came from the mountains between Turkey and Siberia, in the last century anything with a vaguely Eastern origin was attributed to Turkey.

Turkey Rhubarb seems to have deeper roots than this in Cornish tradition, see the description of the St Ives Guise Dance on the Federation of Old Cornwall Societies Christmas Web Pages:

The Turkey Rhubarb dance always marked the finale to the evening’s proceedings, after which the exhausted participants at last gave up and went home. This dance was also associated with the Christmas Mummers play, where a concertina, sometimes referred to as a ‘cordial’, would provide the music. The dancers performed in heavy shoes fitted with scoots, metal pieces attached to the soles.

They commented further:

The name ‘Turkey Rhubarb’ is itself a delightful enigma. There might obviously be some connection or confusion with the Turkish knight in the Mummers Christmas play. However, Turkey Rhubarb was not the common garden rhubarb, parts of which are highly toxic. It was rather a Chinese herb, ‘Rheum Palmatum’. Apothecaries used its root as a cure for diarrhoea, but its use can cause intense cramping. Larger doses were employed as a laxative. Morton Nance’s Cornish Dictionary gives ‘Tavol Turkey’ is an alternative Cornish word for Rhubarb. Perhaps the antics of the dancers were akin to the cramps effect on the body and someone made the humorous [?] connection.

Jon Boden sang Turkey Rhubarb as the March 19, 2011 entry of his project A Folk Song a Day.

Lyrics
Tim Hart and Maddy Prior sing Turkey Rhubarb
Turkey Rhubarb, Turkey Rhubarb, Turkey Rhubarb I sell
I come here from Turkey to make you all well
Don't you all know me, oh my name it is Dan
For I am the celebrated Turkey Rhubarb man

LFF