Huanglongbing
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Figure 2
The Asian Citrus Psyllid
The Asian Citrus Psyllid (Diaphorina citri; Figure 1) carries the pathogenic bacterium (Candidatus Liberibacter asiaticus: CLas) causing the disease Huanglongbing or citrus greening. ACP was reported for the time in Florida in 1998 and in California in 2008 and is now well established in the US. ACP females feed on citrus flush and lay tiny eggs that hatch into nymphs and in turn molt into adult psyllid. Upon feeding, the nymphs remove sap from the phloem and excrete honeydew but also produce a characteristic waxy tubule from its back end (Figure 2). CLas is acquired by the ACP adults or nymphs from an infected tree host after feeding events and the bacterium multiply within the insect host. ACP transmits the pathogen when feeding on new trees. Controlling insect vector populations is needed to successfully manage HLB.
Disease Symptoms
HLB can affect most citrus cultivars, species, hybrids and even some citrus relatives. Once trees become infected with CLas, the bacterium multiplies and moves systemically via the phloem element throughout the tree. Early symptoms include yellow shoot (Figure 3). Leaves of infected trees also exhibit asymmetrical blotchy mottling (Figure 4). As a result of infection phloem sieve tubes become occluded and the integrity of the tree is compromised due to blockage of carbon photo-assimilates from the leaf to the fruit and root system. This results in the accumulation of starch in leaves and depletion of starch in roots and feeder root loss is often observed in declining trees (Figure 5 from Nehela and Killiny, 2020). Blockage of carbon transport leads to thinning of the canopy and wood dieback (Figure 6). Affected trees also become more vulnerable to biotic and abiotic stresses causing a faster decline.
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Figure 3
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Figure 6
Fruit from HLB-infected trees are small, poorly colored, lopsided (Figures 7 from Chen et al., 2017), and contain aborted seeds. The fact that fruits retain partially or completely their green color when mature is the reason for the common name 'citrus greening disease' (Figure 8). The juice from affected fruit is low in soluble solids, high in acids and abnormally bitter. This fruit is of no marketable value because of poor size and quality. Overall yield is significantly decreased because affected trees experience heavy fruit drops before fruits can reach full maturity (Figure 9). Since HLB was first discovered in Florida in 2004, the production of citrus has dropped from 292 million boxes in 2003-04 to 41 million boxes in 2021-22 (2023 National Agricultural Statistics Service, NAAS).
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Figure 9