322 G. McCOLLUM AND E. BALDWIN
potential impact of HLB was so significant in Florida that an article
on the disease was published in Scientific American (Kuchment 2013),
along with numerous other articles in the popular press, demonstrating
concerns from a societal perspective about availability and price of cit-
rus fresh fruit and juice. Currently, HLB is arguably the most significant
crisis facing any important food crop.
A. Symptomatology
HLB is difficult to diagnose on the basis of symptoms, especially in the
early stages of disease. There are numerous symptoms of HLB, and cit-
rus trees are often affected by other problems that can result in sim-
ilar symptoms (Bove 2006; Gottwald 2010). The problem is that HLB ́
is also a very non-uniform disease, especially early in development,
within individual trees, within orchards and within production areas
(Gottwald 2010). Early in HLB development, disease symptoms are typ-
ically present only in individual branches or in sectors within a tree.
HLB symptoms are also non-uniform throughout the year, with visi-
ble symptoms less obvious during warm parts of the year and more
pronounced during the cooler months (McClean and Schwarz 1970)
increasing the difficulty of visual diagnostics. HLB may first appear as
one or several completely chlorotic “yellow shoots.” The English trans-
lation of HLB is yellow shoot or yellow branch disease (Bove 2006). ́
With time, the yellow shoots may grow into larger chlorotic branches.
In later stages of the disease, the entire canopy may exhibit severe
chlorosis.
One of the most characteristic symptoms of HLB is an irregular
pattern of leaf chlorosis referred to as “blotchy” mottle (Bove 2006; ́
Gottwald 2010). Leaves with blotchy mottle have several shades of yel-
low, pale green, and dark green. These shades blend into each other, and
there are no sharp boundaries between them. Blotchy mottle is synony-
mous with HLB and accompanies HLB wherever it occurs. Zinc, man-
ganese, or magnesium deficiencies also produce a mottled pattern of
chlorosis in citrus leaves, but with a symmetrical pattern unlike the
asymmetric pattern of HLB blotchy mottle. In the early stages of HLB
within a tree, blotchy mottle may affect large, well-developed leaves,
and may be the only visible foliar symptom. As HLB progresses, foliar
symptoms of zinc deficiency may develop, and such leaves will remain
small, and with time, the whole leaf blade may ultimately turn uni-
formly yellow. Blotchy mottle appears typically on sweet orange, grape-
fruit, and pummelo leaves, but can also be seen on other citrus scion
types. Leaves of lemon and lime also show blotchy mottle, but of a type