Huanglongbing

Huanglongbing (HLB) is a major disease of citrus that has caused catastrophic damage to citrus trees worldwide. The disease causes reduced fruit quality and yield, tree decline, and eventual tree death.

Symptoms are variable and can resemble several disorders of citrus. Typical symptoms include:

• yellow shoots with pale green and yellow flushes.
• non-symmetrical mottled leaves (shades of yellow and green on either side of the mid-rib).
• thickened, leathery leaves.
• enlarged, corky mid-ribs of leaves.
• leaves with zinc deficiency symptoms that include upright leaves in relation to the shoot (acute shoot-leaf angles).

Defoliation, fruit drop, and shoot dieback occurs in more advanced stages. Young trees may die soon after infection, whereas older trees may die in seven to nine years after infection.

Fruit symptoms include small, misshaped fruit that are lopsided or asymmetrical and exhibit color inversion from yellow to orange to green on the peduncle side while remaining green on the stylar end. The vascular tissue is brownish at the peduncle side of fruit. Seeds of affected fruit are small, brown, and aborted.

Huanglongbing has a complex pathosystem (an ecosystem based on parasitism). There are multiple strains, diverse hosts, several insect vectors, and different environmental conditions that affect the expression and spread of the disease. Three forms of the disease are known (Asian, African, and American), and these are associated with different species and strains of Liberibacters that are disseminated by different species of citrus psyllid insect vectors. The pathogen currently found in the US is Candidatus Liberibacter asiaticus. Most citrus species, cultivars, and hybrids are susceptible to the disease. Currently there are no resistant varieties of citrus that have edible fruit.

Vectors of the Liberibacters include citrus psyllids Diaphorina citri (Asian citrus psyllid) and Trioza erytreae (African citrus psyllid, not found in the United States). Huanglongbing can also be spread by grafting. Transmission can occur in the nursery and in the orchard.

Non-citrus hosts have been identified as alternate hosts for the Liberibacters and the citrus psyllids, but their role is still unclear in the epidemiology of the disease.

Maintaining trees in good health, especially root health, is important for trees to withstand huanglongbing. There is no cure for the disease.
California is currently under a huanglongbing eradication program. If ACP nymphs test positive for the huanglongbing bacterium, treatment of the infestation with conventional insecticides is mandatory. If a tree tests positive for huanglongbing via PCR test, treatment of the corresponding infestation with conventional insecticides and removal of the infected trees is mandatory.

For additional information and resources see the Voluntary Grower Response Plan for Huanglongbing.
The CDFA has survey crews that sample psyllids and leaf tissue across the citrus-growing regions of California which they submit for testing in regulatory labs.

In regions of California where Asian citrus psyllid (ACP) is established (southern California) or where ACP Is occasionally reported (San Joaquin Valley), growers can submit their own samples of leaves or psyllids (unless the samples come from a HLB quarantine zone) to the Citrus Pest Detection Program (formerly the Central California Tristeza Education Agency). There is a protocol for preparing samples for submission, so contact the CCTEA prior to collection. Improperly prepared samples may be rejected from the lab. The lab can also test for citrus stubborn, which can have similar symptoms in the foliage of infected trees.
It is best practice to sample leaves from the edges of a citrus orchard for testing because ACP tend to congregate there.
In huanglongbing quarantine zones, all sample collection must be done by regulatory authorities.
Dogs are being used as a potential early detection technique to identify trees that have been potentially exposed to the bacterium that causes huanglongbing. Neither dogs nor any other early detection techniques are currently recognized by the USDA as approved tools to confirm the presence of Candidatus Liberibacter asiaticus.