Candidatus Liberibacter asiaticus
Why is it problem? Huanglongbing (HLB), is one of the most devastating citrus diseases globally. There is no known cure for the disease. It probably entered Bhutan in the 1960s and has become the most serious threat to citrus production. Huanglongbing infection results in reduced yields, poor fruit quality, premature tree death and increased production costs.
Where and when is it a problem? Huanglongbing is present in almost all citrus growing Dzongkhags of Bhutan. Disease incidence and severity is high at elevations below 1200 masl, where the vector Asiatic citrus psyllid is common. However, the disease has also been detected in mandarin trees planted above this elevation.
Typical symptoms produced by HLB are:
- The presence of yellow shoots that stand out among the usually green-looking shoots.
- Initial leaf symptoms include ‘blotchy mottle’ consisting of asymmetrical patches of pale green or yellow.
- Shoot dieback as the disease progresses. This can occur across the whole tree. It may appear as yellow with sparse foliage. Mottled leaves may turn yellow and become leathery. Veins may become corky.
- Trees may produce small and upright leaves.
- Nutrient deficiency symptoms, such as Zinc deficiency, may become apparent.
- Fruits of infected trees may exhibit ‘colour inversion’ manifested as abnormal colouring from the stylar to the penduncular ends.
- Fruits may also become lop-sided. This is most apparent when fruits are cut open parallel to the fruit axis.
- Infected fruits may also have aborted seeds which appear as brown and shrivelled seeds when fruits are cut open.
Confusion with other problems: HLB symptoms may be confused with symptoms produced by other factors. For example, Phytophthora root/trunk rot and trunk borer can cause yellow shoots and leaf mottling. Powdery mildew infection can leave trees with severe shoot die-back that could be mistaken for die-back resulting from HLB infection. Zinc deficient leaves also appear mottled but it is different from blotchy mottle. Mottling due to Zinc deficiency are presented as symmetrical chlorosis on both sides of midribs and exhibit interveinal chlorosis (i.e., the veins do not become yellow).
Lifecycle: In Bhutan, HLB is caused by a putative species of phloem-limited α-Proteobacteria that has not been cultured: ‘Candidatus Liberibacter asiaticus’. The bacterium multiples and spreads within the phloem of the plant. The psyllid vector can pick up the bacterium if it feeds on the phloem for at least 15 minutes. However, the longer the psyllid feeds on an infected host the more likely it is to pick up the bacterium. Symptoms can take between one and three years to become apparent, sometimes faster in young grafted plants.
Transmission: Natural transmission is likely to only be possible through the Asiatic citrus psyllid. However, the pathogen can also be spread by grafting with diseased budwood, In Bhutan, planting of already infected materials seems to be the cause of occurrence of HLB at higher elevations. Ad hoc collection of germplasm either from the wild or from orchards also aggravate the problem as such plants could harbour the bacterium. Seed transmission probably does not occur.
When can damage be expected? In Bhutan HLB will thrive where ever its’ vector, Asiatic citrus psyllid, is common. The vector is common to about 1200 m asl, and present up to about 1450 m asl. The rate of disease spread will increase with psyllid density.
Hosts: Several citrus species and hybrids, and citrus relatives are known hosts. Some non-citrus plants are also known to harbour the bacterium. These include periwinkle/vinca (Catharanthus roseus), and dodder (Cuscuta australis, Cuscuta campestris and Cuscuta pentagona).
HLB is a devastating disease. Citrus production up to about 1200 m asl will require a rigorously applied integrated management approach. At higher altitudes strict quarantine measures will still need to be applied to ensure that orchards remain disease-free.
Regular monitoring is necessary to track the spread and development of the disease through the orchard, and also to assess the abundance of the vector. Formal monitoring of nurseries for the presence of both the bacterium and the vector is needed to guarantee stock is disease-free (see below).
Effect of variety
This pathogen may persist and multiple in many citrus species. Disease severity can differ between Citrus species and hybrids. Mandarins are among the most susceptible. Studies have found variation in susceptibility between cultivars of mandarin and oranges.
- Effective management of HLB in Bhutan is dependent on the movement and supply of pathogen-free nursery stock
- Ideally nursery stock should be produced in areas demonstrated to be free of both the disease and its vector. This is most likely to be at high altitude locations (> 1200 masl).
- HLB-affected stock must be destroyed immediately as there are no techniques for sterilising plants once they are infected. They must not be moved or supplied.
- Nursery operators need to demonstrate that stock is disease-free by using sterile techniques of germplasm production. Additionally, samples must be tested for the bacterium every quarter in collaboration with NPPC. Zero tolerance must be observed.
- Researchers should avoid collecting germplasm from the wild as they may be infected.
- Farmers should not collect or share seedlings among themselves.
Orchards up to about 1200 masl.
- Most orchards below 1200 masl are already infected with HLB. Management should therefore start by progressively removing the infected trees and replanting at high densities with certified pathogen and psyllid free materials. If trees are cut off, then ensure the tree stump is also dead by treating the stumps with glyphosate, as any regrowth will continue to be a disease source. Progressive removal involves regular monitoring for signs and symptoms of the disease. As the pathogen is already widely distributed in Bhutan, laboratory confirmation for its presence is not required at farm level for orchards below 1200 masl.
- Vectors also need to be managed, see Asiatic citrus psyllid. This can help slow, but not prevent, the spread of the disease within an orchard. Area-wide vector management is essential in places where many orchards are present close to each other.
- When pruning trees, disinfect tools between each tree with 4% bleach (or undiluted household bleach e.g., Robin fabric stain remover liquid bleach). Note that pruning HLB symptomatic branches or shoots as part of management is not effective as the bacterium is systemic within the plant.
- Abandoned orchards should be destroyed to prevent them from being a local source of disease.
- All alternative hosts must be removed from within and in close proximity to orchards before pathogen-free trees are planted. This includes Bergera koenigii (curry leaf plants) and Murraya paniculata and Murraya elongata These plants (especially Murraya plants) can sustain the vector, Asian citrus psyllids, even when diseased citrus plants fail to produce young flush. This can allow the population to quickly re-establish within the orchard as soon as young citrus plants with new flush (or even mature trees with new flush) become available. Moreover, curry leaf and Murraya are known to be transient hosts of the HLB pathogen.
Orchards above about 1200 masl.
- Citrus production should be viable at higher altitudes (about 1200 to 1700 masl) provided varieties suited to high altitudes are used.
- Strict quarantine need to be practiced, especially to ensure all new stock is guaranteed to be free of HLB.
- Regularly inspect the orchard for HLB symptoms, and rogue out diseased plants.
There are no chemicals that can control HLB. However, insecticides can be used to suppress Asiatic citrus psyllids in orchards up to about 1200 m asl. This can help slow the spread of the disease within and between orchards.
Image acknowledgements: NPPC.