Huanglongbing (HLB, meaning 'yellow shoot disease'), is one of the nine citrus diseases known to be spread by both grafting, and natural contamination through insect vectors. In the past decade, the disease has gained pandemic status as a result of man’s activity, possibly combined with climatic change, thus allowing vector and pathogen alike to move long distances and threaten the major citrus producing areas of the world.
Old descriptions in Assam and the Punjab are suggestive of HLB. However, the first true outbreaks in orchards were reported by the turn of the 1940s in the Chinese province of Guangdong—hence the official Chinese name of the disease. This was followed by separate outbreaks in the mid-1960s in the Maharashtra (India) and in the Transvaal (South Africa). The disease was doubtless already present in these regions but the possibility of it spreading by grafting and by leaf sucking insects was as yet unknown.
In addition to propagation by budwood in nurseries, the disease was found to be transmitted by two psyllids, one Asian (Diaphorina citri) and the other African (Trioza erytreae).
Citrus Psyllid F168 p41) The symptoms are identical in both cases: asymmetrical blotchy mottle of leaves, with regard to the main leaf vein; the fruits are also lopsided, weakly coloured with a low juice or sugar contents and aborted seeds, they display typical inverted coloration with initial change around the peduncular end whereas on healthy fruits it starts at the stylar end.
2) In both cases, the canopy of infected trees exhibits yellow shoots followed by twig dieback and gradual decline of trees. Healthy plants established in open orchards submitted to psyllid invasions, develop twig sectorial dieback from top downwards. Infected fields often lose any profitability within five years after planting.
Map of the different forms of HLB F168 p41) Because the African form of HLB is sensitive to high temperatures and in sub-Saharan Africa the symptoms disappear when the total of degree-hours greater than 30°C is 800 or more. Furthermore, the upsurges of African psyllid require cool and moist conditions. Eggs and first instar nymphs cannot stand hot, dry air (air saturation deficit of 30 millibars or more). As a result, the African HLB is generally found in orchards at elevations greater than 600-700 meters, although these are known to give high-quality fruits. Indeed, the oranges and mandarins grown on the high plateaux are better coloured and more tasty.
2) In contrast, the Asian citrus psyllid and the Asian form of HLB can withstand extreme temperatures ranging from freezing in the winter, to very arid and warmest summers in the world.
Affected citrus F168 p4
Psyllid mummies F168 p4Entomologists estimate that the psyllids appeared during the Gondwana period 150 million years ago, and that larval cycles of these insects adapted gradually to narrower niches on preferential host plants. Furthermore the age of HLB bacteria was evaluated by molecular bio-chronology, and estimated to the Gondwana period also. Contact between T. erytreae or D. citri and cultivated citrus is much more recent as the latter only appeared some 4 million years ago in Australasia and was subsequently disseminated by man. As an example, a 'relic area' like the Comoros islands harbours a very special Diaphorina that has adapted to the citrus 'newly' introduced there.
Map of the different forms of HLB F168 p4This is the case in the Mascarene archipelago (Réunion, Mauritius and Rodrigues) and the south-eastern part of the Arabian peninsula, where both citrus psyllids are present. Their ecological niches may occasionally overlap. Furthermore, the African psyllid can alternatively carry the Asian form of HLB and the Asian psyllid the African form. Not surprisingly both bacterial types have been found colonising a same tree in Reunion.
sleve tube F168 p4A provisional mapping of expected vector spread may be obtained by computing temperature and relative humidity data. For example, Trioza erytreae would be able to colonize Mediterranean coastal areas, with egg laying and larval development periods in the spring time, and adults over-surviving the other seasons. The larvae of this psyllid are nesting in galls on the underside of leaves and their transport on long distances will occur on rooted planting material. Then adults, with fairly good flight capability, will spread the species in the newly contaminated territories.
As to Diaphorina citri, the larvae are mobile with possible survival on young twigs or fruit stems. The Asian citrus psyllid will therefore spread through the transfer of cut planting material, e.g. leaves or fruits.
Yellow sticky cards in well-chosen places and observed regularly can be used as a warning network, since both psyllids are attracted by this colour especially the 'Saturn Yellow' type.
Temperature RH F168 p4The first signs on leaves can be confused with mineral nutrient disorders especially zinc or manganese deficiencies. However, a true physiological deficiency will affect an entire orchard and the pattern of yellow shoot on canopy trees will tend to be evenly distributed. In contrast, symptoms of HLB are much more random and sectorial for both individual trees or contaminated orchards. Another possible confusion may result from Phytophthora attacks related to soil water logging. However, in this case the leaf symptoms begin with yellowing along the veins.
It is therefore important to back up field visual diagnostics by laboratory assay. Once such a confirmation has been made, visual detection is carried out by specially trained officers who inspect the canopy of the trees. Towed platforms giving a view of the canopy are used, and remote sensing techniques can be envisaged although not yet operational.
Both forms of HLB are associated with Gram- bacteria that proliferate in the vascular system conveying the elaborated sap (primary phloem and liber). This translocation occurs in elongated nucleus-free cells in the form of tubes, equipped with sieves pores. This vascular system feeds the biological active parts of the plant, especially buds, fruits and roots. On HLB affected tissues sieve pores plugging hinder sap transfer.
These bacteria were given the name Liberibacter because they are found in liber tissue. However, the isolation in pure culture of these organisms has not yet been achieved, although their 16S ribosomal DNA were recently identified. They are thus classified in the alpha-proteobacteria group and named provisionally Candidatus Liberibacter asiaticus and Candidatus Liberibacter africanus.
A French INRA-IFAC team played an important role for detecting the Asian and African HLB organisms at the end of the 1960s. It identified for the first time the presence of bacteria on affected orange leaves from India, Reunion island and South Africa, using transmission electron microscopy.
Detection techniques with DNA probes and PCR primers are available today, making it possible to evaluate the bacterial load in both the tissues of contaminated citrus plants and the body of psyllid vectors. This applies to both the African and Asian strains of HLB.
But to different degrees. Oranges mandarins (together with tangelos, tangors or other easy peelers) and grapefruits, are among the most susceptible. Lemons, limes, kaffir limes, citrons and calamondins display less drastic symptoms in spite of hosting bacterial loads as high as in the former group. So far no rootstock was found to provide resistance to HLB. Acid citrus fruits (rough lemons, limes, calamondins, etc.), generally grown from seed, are common backyard trees in the villages of Africa and Asia. They form multiple spots for infectious agents and vectors. The same applies to certain ornamental rutaceous plants such as the species Murraya paniculata, a preferential host plant of D. citri and very commonly used in landscaping.
A territory may harbour citrus psyllid populations without necessarily the HLB bacterium being present. However, such a territory becomes extremely vulnerable as the importing of contaminated budwood or plants, can cause the rapid spread of the disease. This scenario was recently observed in Brazil where D. citri had been introduced accidentally by the mid-twentieth century, but without the HLB bacterium. Some fifty years later, the probable importing of contaminated budwood and other circumstances that have not yet been elucidated, caused a sudden outbreak of the disease in Sao Paolo state.
In northern America HLB arrived in Florida most probably by the mid-1990s with ornamental plants hosting both the disease and D. citri larvae. For some time the HLB spread unknowingly in peri-urban ornamental plant nurseries, before appearing suddenly in commercial orchards.
Epidemiologists agree that among citrus diseases, HLB develop the most dangerous epidemics, irrespective of smallholding orchards or large intensive plantations.
For the psyllid vectors, the HLB organism which is part of the symbiontic flora in the digestive tract, is conveyed through the haemolymph and the salivary glands. The leaf sucking mouthpart of the insect comprises a stylet which is used to pierce and suck the fluid from the leaf. The stylet has three fine threads sliding jerkily against each other for piercing the leaf tissue up to the phloem. Gustatory sensillas mediate acceptance or rejection of plant fluid thus playing a major role in locating tissue feeding.
To sum up, the spread of HLB by vector first involves feeding on a contaminated plant. The HLB bacteria then multiply in the tissues of the insect to the salivary glands for inoculation during subsequent feedings. Research is in progress to gain better knowledge of these mechanisms and especially the translocation-multiplication of the pathogen within the vector.
Host relashionship F168 p4The spread of the disease from an initial outbreak depends on vector population density and the prevalence of HLB bacteria and hence the frequency of pathogen-vector contacts. As regards the 'pathogen' parameter and in the light of the explanations above, strict regulations for propagating healthy planting material in nurseries are of prime importance. This leads to setting up certification schemes in which the production of healthy budwood, and the production of young grafted plants are performed under insect-proof greenhouses.
The rate of the spread of the disease in orchards depends on environmental conditions and cultural practices. For example, D. citri is omnipresent in Asia, where the eradication of HLB is socially difficult since millions of families own small private orchards or backyard trees. The lifetime of orange and mandarin plantations is hardly more than five years. Citrus growers compensate for this short production period by using extremely high density planting systems that require intensive labour.
HLB was first observed in Brazil in March 2004 and it spread rapidly to some 200 municipalities in the Sao Paolo state citrus belt. Vast orchard areas are planted there, with large orchards of 100 000 and even up to 3 million trees. The eradication of the first outbreaks involved the destruction of 5 million out of a total of 150 million trees following monthly inspection from mobile platforms. With continuous insecticide cover, it has been found that because of incubation time lag, the percentage of trees actually infected at a given moment is in fact twice as great as the number of trees displaying the very first symptoms of the disease.
This type of strategy has only one chance of success. The psyllid populations must be initially established without their natural enemies, and the introduction of primary ectoparasites must be introduced with previous careful elimination of secondary and tertiary parasites. This method proved its worth in Réunion in the 1970s thanks to the efforts of a IRFA-INRA-IRAT and SRA joint team. It resulted in the eradication of T. erytreae, and achieved a very strong decrease in D. citri populations. As healthy planting material was regularly secured, a gradual disappearance of HLB was obtained without the need for a massive and hazardous total eradication of affected citrus trees. Effective biological control of D. citri has just been repeated successfully ten years ago in Guadeloupe by a joint operation INRA and CIRAD.
Biocontrol F168 p8Economic studies in Florida have shown a rapid production decrease due to HLB, in the absence of stringent control strategies. This includes early diagnostic and destruction of infected trees followed by eventual replanting with clean material. This prophylactic measures result in an increase of production cost of at least 35 to 50%. Such an extra cost results from the purchase of healthy plants, detection costs (five or six scouts per year), replanting, the increase in insecticide spraying, and the temporary loss of production in eradicated and replanted fields. Outdoor field-grown the nurseries are forbidden and must operate now under insect-proof greenhouses. In addition, care must be taken in the removal of contaminated trees to prevent contamination by regrowth of stolons infected by HLB. It is important to coat the trunk with herbicide just after felling and cover it with plastic sheeting.
Many growers have been discouraged by these constraints, especially near urban areas exposed to landowning speculation. HLB and D. citri are beginning to reach dangerous levels even in the remote Everglades orchards.
Floride F168 p8Intertropical islands used often as natural quarantine facilities over the previous centuries were and still are critical step for the spread of vector-borne citrus diseases such as HLB. Many islands harbour germplasm repositories of citrus and/or ornamental Rutaceae. In addition to the famous case of the Mascarene archipelago hosting both psyllid species, T. erytreae appeared in Madeira and the Canary Islands about ten years ago. Fresh cut twigs of Murraya (carry plant) bearing D. citri larvae were exported recently from Hawaii to California. D. citri appeared also in the southern-most island group of Japan stretching towards Taiwan, and in Timor island near Australia.
All these places are served by daily flights, with all the associated risks.
Organization F168 p8The Mediterranean area, Japan and Australia are still free of HLB for the moment.
The Mediterranean basin, producing 17 million tonnes of citrus each year, is clearly most at risk, given the scale of tourist travel, the importance of ornamental plants and the inter-twin situation of orchards with ornamental citrus. Citrus is grown in small family orchards often at the edge of urban and tourist zones for reasons of water competition. The threat comes both from the East with the presence of D. citri and Asian HLB in the Middle East on the one hand, and from the West with the arrival of T. erytreae in Madeira and the Canaries on the other.
Concerted preventive strategies run by regional multi-disciplinary teams are essential for launching relevant preventive scheme. Today, it does not seem that measures are being taken to match the threat.
photo F168 p9The research conducted by French teams is still a reference but should now be structured in a Euro-Mediterranean framework. CIRAD has newly resumed research work with a programme headed by the FCPRAC (Florida Citrus Production Research Advisory Council—see box).
The economic and social issues are considerable as the Mediterranean basin alone produces some 18 million tonnes of citrus, and trades nearly 6 million tonnes with a market value that probably amounts to between 5 and 10 thousand million euros.
Bernard Aubert, consultant Adac-Cirad
Click "Continue" to continue shopping or "See your basket" to complete the order.
