Agriculture: Citrus Pest Management Guidelines

Huanglongbing

  • Candidatus Liberibacter asiaticus
  • Symptoms and Signs

    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.

    Comments on the Disease

    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.

    Management

    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.

    Common name Amount to use REI‡ PHI‡
    (Example trade name) (hours) (days)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide‚Äôs properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.
    PREHARVEST
    A. OXYTETRACYCLINE
      (Fireline 17WP) 1.5 lb/acre 12 40
      MODE-OF-ACTION GROUP NAME (NUMBER1): tetracycline antibiotic (41)
      . . . OR . . .
      STREPTOMYCIN
      (FireWall 50WP) 11 oz/acre 12 60
      MODE-OF-ACTION GROUP NAME (NUMBER1): glucopyranosyl antibiotic (25)
      COMMENTS: For use on all susceptible citrus. Use oxytetracycline and streptomycin in rotation. Apply aqueous dilutions to trees when new growth occurs to protect flushes of new leaves against transmission of the huanglongbing pathogen Candidatus Liberibacter asiaticus by ACP. Retreatment interval is a minimum of 21 days. Maximum number of applications is three per year of each antibiotic, used in rotation, for a total of six applications per year. Total amount of product per year is 4.5 lb and 2.1 lb of FireLine and FireWall, respectively, or no more than 0.76 lb and 1.35 lb a.i. per year, respectively. Typical timings include three to four spring (Feb. through May) and two to three fall (late August through October) applications.
    Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases, the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
    1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of action. Fungicides with different group numbers are suitable to alternate in a resistance management program. In California, make no more than one application of a fungicide with a mode-of-action group number associated with high resistance risk before rotating to a fungicide with a different mode-of-action group number; for other fungicides, make no more than two consecutive applications before rotating to fungicide with a different mode-of-action group number.
    Text Updated: 07/23
    Treatment Table Updated: 07/23
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