330 G. McCOLLUM AND E. BALDWIN
appearance of HLB symptoms. The progression of HLB symptom devel-
opment was correlated with an increasing degree of ultrastructural aber-
rations. Curiously, CLas cells were most abundant in sieve tubes in
presymptomatic tissue from flushes but were not seen in highly symp-
tomatic mature leaf samples; similar results were reported by Fan et al.
(2013). In contrast, Shokrollah et al.’s (2010) TEM detection of HLB has
shown that the number of bacteria in sieve tubes is higher in leaves with
strong mottle than those without symptoms or mildly symptomatic.
According to PCR analysis, the greatest amounts of CLas 16S rDNA
are always found in the most HLB-symptomatic leaves (Bov ́e 2006;
McCollum et al. 2014).
B. Phloem Transport
Based on anatomical observations, a commonly accepted hypothesis
is that as CLas titer increases in infected tissues, occlusion of sieve
pores and phloem cell collapse blocks translocation resulting in accu-
mulation of soluble sugars in tissues acropetal to the blockage, leading
ultimately to accumulation of starch (Schneider 1968; Etxeberria et al.
2009; Achor et al. 2010; Fan et al. 2010; Aritua et al. 2013; Brodersen
et al. 2014). Indeed, in pulse chase experiments, it was demonstrated
that a greater amount of labeled carbon was retained in CLas-infected
leaves than in leaves free of the pathogen (Koh et al. 2012). Experiments
with the fluorescent dye CDFA, used to follow movement in phloem,
have shown inhibition of export from leaves infected with CLas com-
pared to leaves free of the pathogen (Fan et al. 2012; Koh et al. 2012).
Although these results suggest that phloem transport is inhibited in
CLas-infected plants, they are by no means conclusive.
Despite the presumed inhibition of phloem transport in CLas-infected
plants, vegetative growth and fruit production can continue, although
both decrease with time after infection. Furthermore, parenchyma cells
in woody stems and bark tissue basipetal from the leaf canopy also
accumulate starch despite the evident degeneration of phloem (Etxeber-
ria et al. 2009). If photoassimilate transport were totally blocked at the
leaf/petiole level, there would be insufficient carbohydrate flow to sus-
tain starch synthesis in a basipetal direction and to support the growth
of developing twigs, lower branches, and fruit.
C. Starch Accumulation
Excessive accumulation of starch in aerial plant parts is the most consis-
tent biochemical response to CLas infection (Schneider 1968; Etxeberria