The Rex Union orange, a cross between a Seville orange and pomelo, a variety unique to SA, is only grown commercially on one farm in the nation, just outside Rustenburg. The varietal was developed by George Wellington Rex. He introduced it on the aforementioned farm, Lemoenfontein, just across the road from the famed Hunters Rest Hotel.
It is said that Rex, 
who missed his favorite breakfast spread, marmalade, selected this natural hybrid by grafting the first plants on the very farm, the original orchard of around 250 trees still stands on the farm.
The fruit is really very bitter and doesn’t make for good eating; rather the orange skin and thick pith is prized for making excellent marmalade. This past weekend marked the annual Slow Food Johannesburg member picnic and picking of the fruit on the farm. The second-generation trees were not quite as laden with fruit as they were in 2020 because of the heavy unseasonal rains the farm experienced earlier this year had destroyed many of the blossoms.
Source: timeslive.co.za