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; and
- 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 Liberibactors 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.
Vectors of the Liberibacters include citrus psyllids Diaphorina citri and Trioza erytreae. 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.
National and international quarantines are critical to minimize the potential of long-distance dissemination. Integrated pest management strategies are needed such as:
- early detection,
- chemical and biological control of the vectors,
- potential chemical control of the Liberibacters using chemical and antibiotic treatments of trees,
- isolation and protection of budwood sources and plant propagation in screened-in, insect-proof locations, and
- development of resistant cultivars.
|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’s properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.|
|(Fireline 17WP)||1.5 lb/acre||12||60|
|MODE-OF-ACTION GROUP NAME (NUMBER1): tetracycline antibiotic (41)|
|. . . OR . . .|
|(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 (HLB) pathogen Candidatus Liberibacter asiaticus by the Asian citrus psyllid (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 0.765 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 actions. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode-of-action group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode-of-action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode-of-action group number.|