Paula Tennant, Contributor
Figure 2: Misshapen fruit of huanglongbing-infected trees. (Source: Dr MaryLou Polek)
THE CITRUS species are susceptible to a number of destructive diseases that are continuously emerging and which can severely limit production or totally decimate an industry.
Apart from soil-borne infections caused by Phytophthora, a post-harvest pathogens that reduce productivity and fruit quality, there are other well-known viruses which include tristeza (a viral disease of citrus trees, usually transmitted on the graft of a sour-orange rootstock and causing a rapid decline or the death of the tree) caused by the Citrus tristeza virus. There is also the crippling bacterial canker (Xan-thomonas axonopodis pv. citri), and the most recent introduction tothe Western Hemisphere, citrushuanglongbing.
Citrus huanglongbing (HLB), also known as citrus greening disease or yellow-shoot disease. Is is one of the most extensively studied bacterial diseases of citrus. HLB is known as the century-old malady of citrus in Asia (the Indian subcontinent, China, and South East Asia) and has a long history in East Africa, Cameroon in West Africa, Madagascar, Réunion and the Mauritius islands, and the southwestern Arabian Peninsula.
In 2004, HLB was recorded in the citrus growing regions of São Paulo, Brazil, in Florida, United States, in 2005, and in Cuba in 2007. The Mediterranean basin, the Middle East, Oceania and Jamaica appa-rently remain free of the disease.
Hlb symptoms
Although it is generally believed that it is difficult to diagnose HLB on the basis of symptom expression, given the close resemblance with other diseases (stubborn disease and citrus tristeza) and mineral deficiencies (e.g., zinc, manganese, calcium, sulphur and boron), some symptoms are highly characteristic of HLB. Infection of young citrus trees is generally marked by the appearance of a yellow shoot composed of blotchy, mottled leaves, which later progresses across the entire canopy (Fig 1).
Yellowing
Young diseased trees never come into production. Disease symptoms in older trees, on the other hand, are at first confined to the initial area of infection, while the remaining canopy shows normal vegetative growth and fruit production; hence, the name 'huanglongbing' or 'yellow dragon disease'.
Severely infected trees develop more extensive yellowing, and are sparsely foliated, stunted and non-productive. Generally, severe symptom expression of dieback and severe decline are more prevalent in Asia than in other regions. Associated with the mottle foliar symptom expression is the premature shedding of green fruit, hence the name 'greening'. Fruits that remain on the tree are small, misshapen, contain aborted seeds and mature only on one side (Fig 2). They are mainly inedible, high in acid content and very bitter.
Greening symptoms of fruits occur mainly in Africa. Sweet oranges, mandarins and mandarin hybrids are most affected by HLB regardless of the rootstock, whereas grapefruits, 'Rangpur lime', lemons, and pummelos develop less severe disease symptoms. 'Mexican lime', trifoliate orange and their hybrids are regarded as most tolerant to HLB, which causes only leaf mottling symptoms on these citrus cultivars. HLB affects other plants of Rutaceae, such as the ornamental shrub, orange jasmine (Murraya paniculata), Chinese box orange (Severinia buxifolia) and cape chestnut (Calodendrum capense).
Causal agent
The causal agent of HLB is a bacterium (Candidatus liberibacter), which is limited to the tissues that conduct nutrients in the plant. Three forms of the pathogen have been described in Asia, Africa, and Brazil, respectively. The disease elicited by these three forms differs by a combination of environmental conditions and insect vectors. HLB infections caused by the heat-tolerant forms in Asia and Brazil are vectored by the insect psyllid, Diaphorina citri. Symptom expression occurs under both warm (27 to 35C) and cool conditions (20 and 24C).
The African form of HLB is a heat-sensitive form that is vectored by the pysllid, Trioza erytreae. Symptoms are expressed only under relatively cool conditions. Heat tolerance of both psyllid vectors corresponds to that of the HLB forms. D. citri thrives in hot, low-lying areas of south and southeast Asia, eastern Iran, Saudi Arabia, Argentina, Brazil, Honduras, Mexico, Texas, Florida, the Bahamas, Cuba, Puerto Rico, Costa Rica and Guadeloupe.
The African T. erytreae prefers cooler conditions at elevations 500m above sea level. The insect psyllid (D. citri) has been present in Jamaica since 2002 and has become well established throughout the island. Adult psyllids resemble small cicadas. They are about 2.5mm long and yellowish brown. The trans-parent wings, with white spots or light brown with a broad, beige, longitudinal band in the centre, are held over the back when at rest (Fig 3). Nymphs and adults are capable of transmitting the etiological agent of HLB.
Control strategies
As with citrus canker, sound-integrated, pest-management strategies are recommended for the management of HLB. Establishment of orchards with disease-free materials, diligent scouting of citrus and non-citrus hosts for the identification and removal of infected branches or trees, and a psyllid control programme employing insecticides or biocontrol agents have proven effective in some regions.
Brazil has been able to successfully to manage HLB through tree removal and insecticide treatments against the psyllid vectors. The country's success has also been attributed to favourable factors, such as the absence of tropical storms and hurricanes, which plague other countries year after year and possibly play a role in the spreading of HLB and the insect vector.
Prevention efforts
Essentially, control is achieved by preventing trees from becoming infected. If HLB is not present in a region, quarantine measures should be enforced to prevent the entry of HLB. If HLB has entered, the first immediate effort is directed towards a survey of the region to determine the extent of the outbreak and the second effort is then the prevention of any additional trees from becoming infected through the removal of the infected trees (the source of inoculum) and by keeping the populations of the psyllids low. Without these measures in place, HLB spreads very quickly. It is reported in China and Brazil that an orchard can reach up to 100 per cent infection within five years. Moreover, the disease causes the death of young trees within eight years after planting. Such losses are significant since citrus plants come into full bearing about eight years after planting and profits are attainable between eight to 10 years.
Monitoring orchards
In light of the distribution of the psyllid vector in Jamaica and the presence of the etiological agent in neighbouring regions, it is perhaps only a matter of time before HLB is detected in Jamaica. It is, therefore, pertinent that citrus orchards are closely monitored for disease symptoms and that appropriate safeguards are established to reduce the effects of this destructive pathogen on the citrus industry. The latter includes the development of rapid methods of diagnosis, the production of pathogen-free planting materials under insect-proof greenhouses, the removal of all infected trees immediately following detection of the pathogen and the management of vector populations. It is important to avoid bringing propagation materials from HLB-infected areas into Jamaica.
n The MIAS is a non-profit organisation of the University of the West Indies, Pure and Applied Sciences Department, offering analytical, technical and web services and specialised science projects. If you have any questions or comments about these articles, please email: mias@uwimona.edu.jm or contact the MIAS Analytical Services Division at 970-2042 or 512-3067 for enquiries on services offered.
Source: www.californiacitrusthreat.com
Figure 3: Adult and nymphs of the psyllid, Diaphorina citri. The psyllid adults and nymphs are typically found on new shoots, along with eggs (which are often nestled in crevices of unfolded leaves).
Figure 1: Yellow-shoot symptoms and blotchy mottle-affected leaves of huanglongbing. Tree seen at right. (Source: Dr MaryLou Polek)
