318 G. McCOLLUM AND E. BALDWIN
to be introduced to Europe, brought by the Portuguese from Southeast
Asia and, for many years, were known as portugals. Grapefruit arose in
Barbados from natural interspecific hybrids between orange and pum-
melo (shaddock) in the 18th century (Gmitter 1995).
Commercial citrus cultivars are clonal propagations of selections
from what were likely ancient natural hybrids or budsports from such
hybrids (Moore 2001). It was not until the early 20th century that hybrid
citrus scions began to be generated through controlled crosses and were
released by Swingle and Webber at the USDA breeding program in
Florida (Soost and Roose 1996). Currently, citrus scion hybrids gen-
erated by controlled breeding efforts still represent only a relatively
small portion of total citrus production. Commercial citrus production
is done using a rather limited number of clonally propagated scions
grafted onto rootstocks that are obtained as nucellar, rather than zygotic
seedlings. In contrast to citrus scions, many important citrus rootstocks
(Swingle, Carrizo) resulted from controlled hybridizations (Soost and
Roose 1996).
Citrus is grown within a broad band of approximately± 40 ◦latitude of
the equator, produced commercially in about 140 countries and ranks
first internationally in trade value among all fruits (Liu et al. 2012).
Sweet oranges represent approximately 70% of total citrus production.
Citrus is grown mainly for two distinct markets, fresh fruit and for pro-
cessed juice, with smaller markets for flavor additives, preserves, and
perfume components. Important producers of citrus for fresh consump-
tion are Brazil, China, United States, EU, Mexico, Egypt, South Africa,
and Turkey. Fresh market citrus fruit generally bring much higher
returns per hectare than do processing oranges. Mandarin production is
concentrated in China, Japan, and other Asian countries, but high qual-
ity export fruit is produced in South Africa, Spain, and other countries
of the Mediterranean Basin. Lemons are produced in Argentina, Cali-
fornia, Italy, Spain, and other Mediterranean countries. Three countries
plus the EU are estimated to account for about 73% of the worldwide
orange production: Brazil (33%), China (14%), United States (13%), and
EU (13%) (USDA, FAS 2015; USDA, NASS 2015). Most U.S. commer-
cial citrus production occurs in four states, Florida (68.7%), California
(27.5%), Texas (2.7%), and Arizona (1.1%). Sweet oranges dominate
Florida’s citrus production (83.6%) followed by grapefruit (12.6%) and
specialty fruit consisting of mostly tangerines and tangelos.
By far, the single greatest citrus type grown is sweet orange for the pro-
cessed juice industry. Two areas dominate the production of oranges for
juice: Sao Paulo State, Brazil (50%) and Florida (35%) (USDA, NASS ̃
2015). More than 95% of Florida’s sweet orange crop is grown for the