Huanglongbing is the most destructive disease of citrus worldwide and has recently been introduced to the US. The disease is caused by the phloem-dwelling pathogenic bacterium Candidatus Liberibacter asiaticus, the asian strain. The bacterium is vectored by the Asian Citrus Psyllid (Diaphorina citri). After mating, female psyllids lay eggs on the new leaf growth of citrus flush, in the folds of unfurled leaves and behind developing leaf buds. Adults and nymphs have been found to both acquire and transmit the pathogenic bacterium upon feeding (Figure 1). Once trees are infected, the bacterium multiplies and moves systemically in the plant. Early symptoms development include yellow shoot (Figure 2). Leaves of infected trees also exhibit asymmetrical blotchy mottling (Figure 3). As a result of infection phloem sieve tubes become occluded and the integrity of the tree is compromised due to blockage of nutrient translocation from the leaf to the root system. Affected trees often die to other biotic and/or abiotic stresses (Figure 4). Fruit from HLB-infected trees are small, lopsided, poorly colored, and contain aborted seeds. The juice from affected fruit is low in soluble solids, high in acids and abnormally bitter. The fruit retains its green color when mature, which is the reason for the common name "citrus greening disease." This fruit is of no marketable value because of poor size and quality (Figure 5). Overall yield is also affected because of significant fruit drops (Figure 6) The HLB bacteria can infect most citrus cultivars, species and hybrids and even some citrus relatives.
Figure 1: ACP eggs on flush, adult insect, and nymphs producing waxy tubules.
Figure 2: Yellowing of branches that gave the name to the yellow gradon disease (a.k.a. Huanglongbing). Note the yellow shoot at the base of the tree.
Figure 3: Asymmetrical pattern of blotchy yellowing or mottling of the leaf, and patches of green on one side of the leaf and yellow on the other side.
Figure 4: Tree decline due to HLB infection (same tree, two years interval). Note the thinning of the canopy with smaller yellowing leaves, and wood dieback.
Figure 5: Impact of HLB on fruit quality with greening and small lopsided fruit.
Figure 6: Impact of HLB on fruit drop. Note the number of fruit on the ground vs. on the tree.