324 G. McCOLLUM AND E. BALDWIN
Fig. 7.3. HLB associated fruit drop.
density for presymptomatic and symptomatic trees, respectively, com-
pared with healthy trees (Johnson et al. 2014).
As HLB progresses, there is a significant increase in preharvest fruit
drop that contributes to the reduction of productivity (Bassanezi et al.
2011). Most HLB-symptomatic fruit either drop prior to harvest, are not
harvested, or are eliminated by sizing equipment prior to entering the
processing stream (Fig. 7.3).
Because citrus is an evergreen perennial species, the severity of HLB
symptoms increase continuously following CLas infection, although
symptoms can fluctuate over the season. Eventually many trees become
so unproductive that they are no longer economically viable. In Florida,
HLB seems to be the most severe in sweet orange, grapefruit, and some
mandarin hybrids. Severe fruit and leaf symptoms can be seen on pum-
melo (C. grandis), even though this species is sometimes erroneously
considered more or less tolerant. Lime (C. aurantifolia) trees can remain
productive even when leaves exhibit characteristic blotchy mottle.
B. Etiology
It is well accepted that HLB is associated with a phloem-limited fas-
tidious훼-proteobacteriumCandidatusLiberibacter sp. Currently, three
species of “Ca.Liberibacter” associated with HLB are recognized based