328 G. McCOLLUM AND E. BALDWIN
did the ACP, perhaps by an introduction of infected budwood, or
ACP that hitchhiked into Brazil from Asia. The relative short time
between appearance of ACP and the appearance of HLB seen in Florida
and Texas, clearly demonstrate how rapidly the pathogen can be
disseminated. In California, HLB was confirmed only 4 years follow-
ing the first ACP find. The time intervals between ACP being found and
the appearance of HLB symptoms on trees is likely a function of the
long incubation period between CLas infection HLB symptom appear-
ance (Gottwald 2010; da Grac ̧a et al. 2016). The combination of increas-
ing ACP finds in California, coupled with the appearance of HLB in
the state suggests that HLB will soon be found in commercial citrus in
California.
Following confirmation of HLB in the Western Hemisphere, research
efforts have increased tremendously. Between 2005 and the time of
this writing, approximately 550 papers have been published on vari-
ous aspects of HLB. Gottwald (2010) reviewed epidemiological aspects
of HLB. Hall and Gottwald (2011) reviewed HLB control strategies.
Wang and Trivedi (2013) reviewed recent literature on HLB from a
microbiological perspective. An extensive review of psyllids and their
role in HLB was published by Grafton-Cardwell et al. (2013). da Grac ̧a
et al. (2015) focused on host plant responses to CLas infection with
an emphasis on plant pathogen interactions between CLas and citrus,
CLas transmission and genetics. Each of these reviews should be con-
sulted for detailed information regarding those topics. For the remain-
der of this review we have chosen to focus on recent research reports
on HLB disease development from a horticultural, plant physiological,
and postharvest quality perspective which covers current approaches
being investigated for therapy of CLas-infected trees to mitigate HLB,
the potential for new citrus varieties not susceptible to HLB, and HLB
impact on flavor and flavor management strategies.
III. CHANGES INDUCED BY CLas INFECTION
Prior to confirmation of CLas in the Western Hemisphere, most pub-
lished research on HLB had been focused on the pathogen, the vec-
tor, and attempts to manage the disease. Only limited research had
been published describing citrus responses to CLas infection other than
visual disease symptoms. Following confirmation of CLas in the West-
ern Hemisphere, research results describing host plant responses to
CLas infection rapidly began to emerge. In this section, our emphasis
is focused on reviewing knowledge that has been gained regarding the
changes induced in citrus trees infected with CLas.