- HUANGLONGBING: DEVASTATING DISEASE OF CITRUS 325
on 16S rDNA sequence: “Ca.L. asiaticus” (CLas), “Ca.L. africanus”
(CLaf), and “Ca.L. americanus” (CLam) (Bove 2006; Gottwald 2007). ́
Transmission of the three Liberibacters is by one of the two psyllids.
CLas and CLam are transmitted by the Asian citrus psyllid (ACP),
Diaphorina citri(Grafton-Cardwell et al. 2013), and CLaf is transmitted
by the African citrus psyllid,Trioza erytreae. So far, neither the African
citrus psyllid nor CLaf have been found in the Western Hemisphere.
Although strong circumstantial evidence indicates that HLB is caused
byCa.L. spp., noCa.L. species has yet been isolated and grown in
pure culture, and as a consequence, Koch’s postulates have not yet been
fulfilled. Studies of bacterial diversity associated with HLB disease sup-
port that CLas is the sole pathogen responsible for HLB in Florida (Tyler
et al. 2009).
Much like the multiple names by which HLB has been known, prior
to 1984, proposed causes of the disease included nutritional deficien-
cies and insect feeding. In the mid-20th century, K.H. Lin, a Chinese
researcher (as cited in Bove 2006) provided a scientific description of ́
HLB and demonstrated its infectious nature. Based on Lin’s findings
that HLB is of pathological rather than physiological or cultural etiol-
ogy, HLB was thought to be of viral origin. Subsequently, microscopy
revealed the presence of what were initially thought to be mycoplasma.
Finally, based on electron microscopy, Garnier et al. (1984) determined
that the pathogen was actually a Gram-negative bacterium. The effects
of penicillin on CLas-infected trees provides additional indirect evi-
dence that CLas is Gram-negative. Penicillin is an antibiotic active
against Gram-negative bacteria, when penicillin was supplied to HLB-
affected, greenhouse-grown sweet orange trees through the roots (Bov ́e
et al. 1980), or to field-grown sweet orange trees by trunk injections
(Aubert and Bove 1980), there was a remission of symptoms. ́
C. ACP, CLas, and HLB in the Western Hemisphere
- ACP, the Psyllid Vector of HLB.ACP was first confirmed present in
the Western Hemisphere in Brazil in 1942. It was not until 1998, that
ACP was found in Florida. The first Florida ACP find was on orange
jasmine (Murrayaspp.), a relative of citrus, in Palm Beach County
(Halbert and Manjunath 2004). Because CLas was not known in the
Western Hemisphere, and the ACP alone is not a pest of economic
significance, no targeted control was attempted in Florida, and ACP
populations increased and spread rapidly. In addition, shipment of
ACP-infested orange jasmine (Halbert and Manjunath 2004) facilitated
movement of the pest throughout the state. By 2000, ACP had been