Breeding “Sweet Oranges” at the USDA U.S. Horticultural Research Laboratory

The sweet orange is the most extensively planted tree fruit in the world.  Sweet orange is an interspecific hybrid rather than a true species.  Cultivars are mutations selected over generations (possibly millennia) of clonally propagating the original hybrid.  Leading sweet orange cultivars have traits that are much-appreciated by consumers, and commercial producers/processors have a hard-won understanding of optimal handling.  Unfortunately sweet orange also has some serious problems, especially high susceptibility to the disease huanglongbing.  Plant breeders have attempted to create improved sweet oranges for decades. However, hybrids from sweet orange crossed with sweet orange reportedly do not produce sweet-orange-like fruit, and breeders reported no sweet-orange-like hybrids despite numerous crosses. In 1989 USDA/ARS released ‘Ambersweet’ [(‘Clementine’ x ‘Orlando’) x Sweet Orange], which was extraordinary in its resemblance to sweet orange. Chemical and organoleptic evaluations resulted in acceptance of ‘Ambersweet’ as a “sweet orange”, and it was widely planted in Florida, but suffered from low productivity. A new generation of sweet-orange-like hybrids are under evaluation, all with ‘Ambersweet’ as a parent.  Volatile profiles were compared to ‘Ambersweet’ and ‘Hamlin’ sweet orange. The profiles of five of the hybrids were closer to ‘Hamlin’ than is ‘Ambersweet’ and are also sweet-orange-like in appearance and from informal sensory panel analysis. One hybrid peels more easily than sweet orange. Several conventional hybrids in the USDA breeding program are displaying apparent resistance to huanglongbing, with some ‘Clementine’ x ‘Orlando’ hybrids among the most promising.  The new sweet-orange-like hybrids are currently being challenged with huanglongbing in replicated trials.