- HUANGLONGBING: DEVASTATING DISEASE OF CITRUS 323
slightly different from that of sweet orange leaves, as the patches of dark
green tend to be much larger than in the case of sweet orange. Blotchy
mottle symptoms also vary among the mandarin varieties that are grown
in different regions of the world.
Following the appearance of chlorosis, affected branches become
stunted, and produce small, erect, “rabbit ear” leaves. Thickened leaves
and corky veins may also be observed. As HLB progresses, there is an
increase in leaf drop, defoliation, and twig dieback. Trees, with both
symptomless and symptomatic sectors, are characteristic of the early
stages of HLB (Gottwald 2010). In severely HLB-affected trees, consid-
erable defoliation is obvious, canopies are very “open,” allowing one to
see into the tree rather than a wall of green leaves.
As foliar HLB symptoms increase in severity, fruit begin to exhibit
characteristic symptoms. Fruits affected by HLB exhibit orange-stained
vascular bundles, are lopsided, smaller than normal fruit, and may con-
tain aborted seeds. In healthy citrus fruits, loss of chlorophyll in the
rind occurs first at the stylar end, and with time, progresses through
the stem end. In HLB-affected fruit, the loss of chlorophyll starts first
at the stem end. HLB-affected fruit often do not develop the full color
observed in healthy fruit (hence one of the names for this disease being
“citrus greening” disease) (Fig. 7.2). Juice from HLB-symptomatic fruit
is similar in quality to juice from immature fruit, with small size,
reduced solids, and high acid (Bassanezi et al. 2009; Dagulo et al. 2010;
Spann and Danyluk 2010; Rosales and Burns 2011). There will be more
discussion on fruit and juice quality later in this review. It has also
been observed that HLB-diseased trees are more adversely affected by
extremes of temperature and moisture than are healthy trees. Conse-
quently, symptoms of stress (e.g., excessive leaf loss and premature
fruit drop) often are more severe in HLB-diseased trees. This stress
intolerance is thought to result, at least partially, from a loss of fibrous
root function (Graham et al. 2013). CLas-infected, 4-year-old “Valencia”
orange trees showed a 30% and 37% reduction in fibrous root mass
Fig. 7.2. HLB symptoms for leaves, tree, and fruit.