- HUANGLONGBING: DEVASTATING DISEASE OF CITRUS 351
non-viable. Management strategies for HLB are more effective on large-
scale plantations, while small orchard management of the disease is
very difficult. Although numerous HLB therapeutic treatments have
been, or are being investigated with the objective of maintaining
productivity of CLas-infected trees, to date none have proven effective.
Development of new citrus scion and rootstock cultivars that are less
susceptible to HLB than are current standards, whether by conventional
plant breeding or genetic transformation, offers the only long-term hope
for a solution to HLB. Unfortunately, citrus breeding is a very long-term
process and creating HLB resistant varieties to replace currently estab-
lished varieties will take time. Some hybrid citrus rootstocks appear
to slow the rate of HLB decline (Albrecht and Bowman 2011, 2012;
Albrecht et al. 2012), however, the value of these rootstocks in com-
mercial production has not yet been confirmed. The most promising
approach for the development of HLB resistant cultivars is through
genetic transformation, as it can be accomplished more rapidly than
can conventional breeding, and genetically modified trees are likely to
retain the desirable characteristics of already established varieties, with
the exception of being less likely to suffer HLB decline. However, cre-
ation and testing of transgenic citrus is a long-term process. In addition,
the regulatory hurdles that will accompany release of transgenic citrus
will be costly. Finally, consumer acceptance of genetically modified cit-
rus may be a problem.
In the grand scheme of things, HLB is a 21st-century version of a sim-
ilar disease that occurred in Ireland in the 1840s. Like citrus, potatoes
are a clonally propagated crop that was being grown as a monoculture.
When the exotic fungal pathogenPhytophthora infestans(causal agent
of potato late blight) invaded Ireland, potato varieties resistant to the
disease did not exist. The impact of potato blight in Ireland was so dev-
astating that it led to the Irish potato famine, and mass exodus of Irish
who had depended on potatoes as a staple of their diets. Potatoes have
four distinct advantages over citrus; first, there is much greater genetic
diversity in potatoes than in citrus; second, potatoes are more amenable
to conventional breeding than is citrus; third, potatoes are an annual
crop and fourth,P. infestansis a topical pathogen that is not insect vec-
tored and can be culturedin vitro. In spite of all this, potato late blight
continues to be a major disease problem for potatoes in many regions.
Based on our knowledge of the HLB pathosystem, our inability to
manage the disease, and the increasing movement of phytopathogens
from regions where they are endemic into new regions suggests that all
citrus-producing areas are at risk from HLB (Bov ́e 2014). It seems clear
that, without the development of HLB resistant citrus, production in