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 (Fig.1). Once trees are infected, the bacterium multiplies and moves systemically in the plant. Early symptoms development include yellow shoot (Fig.2). Leaves of newly infected trees also develop a blotchy mottle appearance. On chronically infected trees, the leaves are small and exhibit asymmetrical blotchy mottling (Fig.3). As a result of infection phloem sieve tubes become occluded and the integrity of the tree becomes compromise due to blockage of nutrient translocation from the leaf to the root system. Affected trees often die to other biotic and/or abiotic stresses (Fig.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 (Fig.5). Overall yield is also affected because of significant fruit drops (Fig.6) The HLB bacteria can infect most citrus cultivars, species and hybrids and even some citrus relatives.
Asian Citrus Psyllid
Figure 1: ACP eggs on flush, adult insect, and nymphs producing waxy tubules. Source: Topics in Subtropics, Spring 2018 Issue, 'An Update on Huanglongbing Disease of Citrus in California'. (Link)
Figure 2: Yellowing of branches that gave the name to the yellow gradon disease (a.k.a. Huanglongbing). Source: P. Rolshausen and Topics in Subtropics, Spring 2018 Issue, 'An Update on Huanglongbing Disease of Citrus in California'. (Link)
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. Source: P. Rolshausen and Scientia, 2018 Issue #117, 'Using the Plant Microbiome in Agriculture'. (Link)
Figure 4: Tree decline due to HLB infection. Source: Citrograph, Fall 2018 Issue, 'Correlating Citrus Tree Health with Microbes'. (Link)
Figure 5: Impact of HLB on fruit quality with greening and small lopsided fruit. Source: P. Rolshausen and Topics in Subtropics, Spring 2018 Issue, 'An Update on Huanglongbing Disease of Citrus in California'. (Link)
Figure 6: Impact of HLB on fruit drop. Source P. Rolshausen