350 G. McCOLLUM AND E. BALDWIN
exacerbated by Diplodia in the grove, and if Diplodia leads to fruit drop
via Diplodia-induced ethylene production.
D. Future Work
There is a need to develop a technology that would either mask or
sequester compounds that are responsible for the HLB disease-induced
off-flavor. Masking could be done by adding volatile or non-volatile
compounds to the juice (Hussein and Barcelon 1991; Szente and Szejtli
2004), while abiding by food regulations that require those compounds
to be isolated from citrus (“from the named fruit flavoring,” FTNF). For
example, fraction composition was drastically different between com-
mercially processed juice versus point-of-sale, freshly squeezed juice
(Bai et al. 2013b). There is a need to investigate a juicing technology
that would extract less bitter compounds from the fruit, or that could
bind and precipitate those bitter compounds.
Since a solution to HLB disease will likely be through HLB-tolerant
or resistant cultivars, developed either by conventional breeding or by
genetic transformation, the search for HLB-tolerance or resistance in
existing citrus types is well underway. One likely candidate is the citrus
relativeP. trifoliata, a member of the Rutaceae family, which is sexually
compatible with the genusCitrus, as described earlier. Poncirus trifo-
liatahas been used as a breeding parent in citrus breeding programs,
but produces unpalatable fruit and, thus was mainly used in rootstock
breeding. However, recent efforts in breeding programs to crossCitrus
×P. trifoliatahas produced some hybrids that exhibit some tolerance to
HLB (Albrecht and Bowman 2011), as well as producing fruit with fla-
vor similar to tangerines or even oranges (Deterre et al. 2013; Bai et al.
2014). Therefore, research is needed to identify hybrids that taste simi-
lar to tangerine and especially orange, with tolerance to HLB disease.
VI. CONCLUSIONS
HLB is without doubt the most serious threat to world citrus pro-
duction and arguably one of the greatest current agricultural crises
today. Although confined to the Eastern Hemisphere until the 21st cen-
tury, with the exception of Mediterranean countries and Australia/New
Zealand, HLB has spread throughout most citrus-producing regions of
the world (Bove 2014; da Grac ́ ̧a et al. 2015). Once an area is invaded
by HLB, the decline in tree productivity and decrease in fruit quality
appears to be inevitable to the point that trees become economically