Huanglongbing (HLB)

Scientific name

[Bacterium] Candidatus Liberibacter spp.

Candidatus Liberibacter asiaticus (Asian form)

Candidatus Liberibacter africanus (African form)

Candidatus Liberibacter americanus (American form)

Other common names

Greening, yellow shoot, yellow dragon

Disease cycle

Huanglongbing (HLB) is presumptively caused by a phloemphloem:
the plant vascular tissue that conducts products of photosynthesis and other organic molecules such as hormones throughout the plant body
limited bacteria. In citrus there are three forms of concern, the Asian, African and American forms. The Asian form of HLB expresses symptoms in both cool and warm conditions. The African form of HLB expresses symptoms only in cool conditions (20-25⁰ C, 68-77⁰ F). Both isolates can be vectored by the Asian citrus psyllid Diaphorina citri (Kuwayana) and by the psyllid Trioza erytreae (Del Guercio). The American form was identified in Brazil in 2004 and is transmitted by D. citri there.

HLB can be graft transmitted but transmission rates are variable because of irregular distribution of bacteria within the hosthost:
an organism that is infected with or fed upon by a pathogenic or parasitic organism
plant. Seed transmission may be possible but studies are not yet conclusive and if it occurs it is only at very low levels and the disease does not seem to persist beyond early seedling stages. The most important method of disease spread occurs by the two species of psyllids that serve as vectors. HLB can be acquired by both nymphs and adults, which can maintain and transmit the disease throughout their 3- to 4-month lifespan. HLB is systemicsystemic:
spreading internally throughout the plant body; may refer to a pathogen or chemical
and has an incubation period of three months to multiple years before symptoms are visible.

Symptoms

Leaf - although symptoms differ according to citrus varietyvariety:
a taxonomic category, a subdivision of species, consisting of naturally occurring or selectively bred populations or individuals that differ from others of the same species in certain minor but heritable traits
, common symptoms may be described. The most characteristic symptom of HLB is a blotchy mottlemottle:
an irregular pattern of indistinct light and dark areas
. This mottling is distinct from nutrient deficiency in that HLB induced mottling usually crosses the veins and is asymmetrically displayed on the leaf blade. Mottling is most frequently found on newly mature hardened-off leaves but fades with leaf age. The blotchy mottlemottle:
an irregular pattern of indistinct light and dark areas
will be visible on both sides of the leaf and have multiple hues of yellow and green. Dark green areas can sometimes be reduced to small circular dark green dots that contrast with the light yellow/green background. This symptom is referred to as green islands and had been occasionally observed on sweet orange. In addition to blotchy mottlemottle:
an irregular pattern of indistinct light and dark areas
, infected leaves may be thicker and leathery and have raised corky veins. It is common to observe foliar symptoms that resemble nutrient deficiency similar to zinc patterned deficiency. A tree affected by HLB may exhibit yellow shoots and or deficiency symptoms that are on one or many branches randomly arranged in the canopy. This contrasts with a true nutrition deficiency that is exhibited uniformly throughout the canopy. On severely infected branches leaves may form "rabbit ears" that are small upright shoots with compressed internodes.

Fruit - fruit may be small and lopsided. Cut fruit may have a curved axis and the vascular columellacolumella:
Any small columnlike structure in various plants and animals, often forming the central axis of development for the organism or an anatomical structure.
can be stained orange-brown. Seed abortion is also common. Fruit may ripen backwards with the stylar end remaining green as the fruit colors. The fruit symptoms with major economic impact are the reduction in fruit size, premature fruit drop, low content of soluble acids in the juice and a bitter or salty taste of the juice

Whole tree - the irregular distribution of symptoms on the tree corresponds with the irregular distribution of the bacteria in the tree. On severely infected trees, foliage may be sparse with the top third of the canopy being thin. Eventually the tree may go into a complete decline, collapse, and die. Trees with a prolonged infection appear stunted when compared to healthy trees.

Regulatory information

The most current regulatory information can be found at:

Regulation documents

Quarantine map

Host range

HLB can infect all citrus cultivars and hybrids and some relatives. Other Genera in the Rutaceae that can harbor HLB include: Atalantia, Balsamocitrus, Calodendrum, Clausena (Wampi), Fortunella (Kumquat), Microcitrus, Murraya (orange-jessamine), Poncirus (trifoliate-orange), Severinia (Chinese box-orange), Swinglea, Toddalia and Triphasia (trifoliate limeberry).

Distribution

The Asian form is found in Asia, the Middle East, South America, Central America, The Caribbean and the Southeast United States.

The African form is found in Africa and the Middle East.

The American form is found only in Brazil.