The work involved 592 trees and indicated that Lam would colonize a citrus tree and the rootstock rapidly, before manifestation of leaf symptoms. Eight months after pruning, symptoms were again present in 62.5% of the 376 trees in which the entire scion was removed 15–20 cm above the grafting line, and in 58.3% of the 216 trees in which only the symptomatic branch was removed, regardless of the variety of sweet orange used as scion , age of the tree , or symptom status of the tree before pruning . The study also revealed latent infections in 7.8% of 79 caged trees which were not showing any symptoms when they were pruned and subsequently expressed HLB symptoms on new flushes. This work served as a basis for the implementation of the laws to enforce elimination of diseased trees, anda three-pronged-system , crucial for success in minimizing the impact of the disease. Recent work involving only Las has provided more detailed information about citrus tree colonization by the pathogen . Las was detected in the roots of potted seedlings of Swingle citrumelo, Cravo Rangpur lime and Sunki mandarin, the main rootstocks used in SPS, some 50 cm below the inoculation site, 45 days after inoculation,mobile vertical grow tables and in the scion and roots of all 4 and 10-year-old orange trees grown on 13 distinct rootstocks. Research subsequently focused on other aspects of the disease epidemiology: impact of ambient temperatures and graft transmission and multiplication of the pathogens in citrus and orange jasmine Jack as incorrectly referred to in other publications. The research on the impact of ambient temperatures focused on exposing Lam+ve and Las+ve trees to distinct daily temperature regimes .

The research was motivated by the already known contrasting responses to ambient temperatures of plants affected by Las or Ca. L. africanus and by the complete lack of information on this subject for Lam. After a series of growth chamber experiments it was demonstrated that Lam is more heat sensitive than Las. Fully symptomatic orange trees affected by Lam exposed to daily regimes of 27 to 32°C, 24 to 32°, or 35 to 38ºC for 60 days were totally cleared of symptoms and of the pathogen, while fully symptomatic trees affected by Las were only partially cleared of symptoms and the pathogen only when exposed to 24 to 38ºC for the same duration. More recently it was shown that this same temperature regime leads to a decline in Las titers in new flushes on symptomatic branches, an impact which would lead to a significant reduction in pathogen acquisition rates by the insect vectors feeding on them . Although field work will add important information on this aspect of the HLB pathosystem, data so far accumulated indicate that high summer temperatures may restrict rates of spread of the disease and help to explain the irregular dissemination patterns of HLB in SPS. Field and greenhouse experiments involving even higher temperatures for different durations were also conducted, with the aim of curing Las+ve trees, but with limited success . The reasons for the limited success were apparently related to the sensitivity of the citrus tree to high temperatures and to the ability of the pathogen to survive in roots. The temperature-time combinations necessary to kill the bacterium were apparently close to those that would kill a citrus tree, and in the roots, the bacterium remains protected from heat. Studies on graft transmission of Las and Lam were conducted with the objective of comparing graft transmission efficiencies and the ability of both bacteria to multiply, individually or simultaneously, in potted Valencia, Hamlin, Pera, and Natal , under conditions favorable for disease development .

Lam was less efficiently transmitted and less able to multiply in citrus leaves of all sweet orange varieties . The percentage of plants that became infected varied from 10.0 to 23.3% for Lam and 66.7 to 73.3% for Las, and the cycle threshold values varied from 24.14 to 24.97 for Lam and 19.42 to 20.92 for Las. These Cts corresponded to average 106 and 107 cells per gram of tissues for Lam and Las, respectively. Similar values were obtained also when field samples, collected from three distinct regions of SPS, were analyzed . No apparent effect of one species over the other was observed in plants inoculated simultaneously with both pathogens. Lower titers of Lam appear to be the main factor explaining its conspicuous decline over the years in SPS. Lower titer would reduce the chances of pathogen acquisition by the insect vector and its consequent transmission to healthy trees, in a pattern similar to the one observed for Las in new flushes exposed to heat . This work also showed that, contrary to Lam+ve plants, Las+ve plants harbored the bacterium at titers close to maximal values, three months before symptom expression, an indication that asymptomatic trees may be serving as a source of inoculum, contributing to dissemination of HLB in the field. The research on orange jasmine aimed at determination of the distribution, based on sampling at 76 urban locations over two time intervals, of orange jasmine trees infected by Lam or Las, and determination of levels of genetic and pathogenic similarities among the orange jasmine and citrus liberibacters, based on sequences of the rplJ gene and on cross inoculation experiments . The work was motivated by the detection of Lam in a single mature orange jasmine tree growing in front of the manager’s house in the citrus farm most affected by Lam in 2004, by the detection of Las in 2005 in orange jasmine trees growing in urban areas and, more importantly, by suspicion that infected M. exotica trees may play an important role in the HLB epidemics.

In the years 2005/2006 Lam was detected in 56 and Las in 2 of the 477 orange jasmine trees from 10 locations and, in 2009, Lam was detected in an additional 5 and Las in 28 of the 309 orange jasmine trees from seven locations. Lam titers were higher in Lam+ve than in Las+ve trees . As happens with infected citrus under favorable conditions for disease development, symptom severity was stronger on the orange jasmine trees infected by Lam than on those infected by Las. The higher symptom severity in M. exotica may not be related only to the higher bacterium titers in this host since in citrus, Lam reaches lower titers than Las. In Lasinfected orange jasmine the infection seemed to be transient. This was observed in naturally infected field trees and in graft-inoculated plants. This work also showed that the infected orange jasmine trees were in locations relatively close to each other and, coincidently, in the area of highest incidence of HLB in citrus at that time, a clear indication of pathogen transmission from host to host by D. citri. Similarity among citrus and orange jasmine liberibacters, in terms of pathogenicity,mobile vertical farm could not be fully determined due to the strong tissues incompatibility observed between citrus and orange jasmine during the cross inoculation experiments. Most budwood used as inoculum died in heterologous combinations. On those plants in which the bud wood survived, only Lam was successfully transmitted and the plants remained infected. Comparative analysis of the rplJ gene from the liberibacters found in orange jasmine with those found in citrus showed that Lam or Las from both hosts were identical. The importance of orange jasmine and citrus as source of Lam to citrus in SPS was investigated in further work involving the insect vector for bacterium inoculation . Higher Lam transmission rates occurred from orange jasmine than from citrus. As orange jasmine trees infected with liberibacter are not systematically eliminated in urban areas, and vector populations not suppressed, orange jasmine may represent a constant risk to neighboring citrus orchards. Also, since nursery production and sale of orange jasmine are not regulated , asymptomatic orange jasmine trees may be important for distributing liberibacters to distant citrus areas still free from the disease. An overview of the HLB epidemics in Brazil, particularly in SPS, and the main research findings on the HLB pathosystem were briefly presented here. Other field work and studies , and the daily experience of the citrus growers with the disease, have confirmed the necessity of eliminating symptomatic trees and controlling the insect vector on an area-wide basis in order to optimize opportunities for successfully minimizing the spread and impact of HLB. Although many research questions still require answers, research has provided a better understanding of the distinct patterns of spatio-temporal progress of the disease, and knowledge required for official responses and establishment of management practices. Among research outcomes, impacts of high temperatures on Las multiplication in new flushes may have some potential for the development of new, less costly and less insectici dedependent strategies to manage HLB. Citrus is an economically important crop worldwide. Citrus Huanglongbing is highly destructive in southern China, particularly in Guangdong and the neighboring provinces. According to literature, HLB was observed in Chaoshan area of Guangdong in the late 1890s, and the disease has remained endemic since then. The infectious nature of HLB was recognized in the 1940-1950s. The association of Candidatus Liberibacter asiaticus with HLB was confirmed in 1996. Based on the infectious pathogen theory, control strategies for HLB control were developed. These include clean nursery stocks, insect vector control and removal of infected trees. While elimination of the HLB pathogen is still a priority for HLB management, there is also high inTherest in how to improve productivity of HLB-affected trees. Current research has focused on heat and soil treatments. Heat treatment was developed in the 1950s, mainly for elimination of HLB pathogen in scions.

We recently studied the effects of high summer temperature on the reduction of Ca. L. asiaticus titers in HLB-affected citrus trees in field. Heat treatment was delivered via covering a tree with a temporary enclosed tent of plastic sheeting, allowing natural sunlight to raise ambient temperature. After treatment three times in summer, significant reductions of Ca. L. asiaticus titers in the treated trees were observed in October, November, and December. In soil treatment, a soil conditioner rich in P, N, K, Mn, and organic matter was added to soil surrounding HLB-affected mandarin citrus trees. We observed that HLB trees showed more vigorous growth and lower titer of Ca. L. asiaticus two months post treatment. Titer reduction was most obvious 7 months post treatment. However, the titer of Ca. L. asiaticus resurged ten months after treatment. We also observed an increase in fruit yield and quality with the soil conditioner treatment.The economically and socially important Spanish citrus industry is totally based on pathogen-free certified cultivars locally produced. In spite of this, Huanglongbing is considered a serious threat, preventive methods being essential to maintain the HLB-free status. With this purpose extensive surveys and routine analyses were conducted in 2009, 2012 and 2013 in the Canary Islands where Trioza erytreae was reported in 2002 . In addition, surveys were also performed in the main citrus growing areas in mainland Spain. In Comunidad Valenciana annual surveys include 100 sampling points in selected orchards and in surrounding areas of 3 ports, 2 airports and 20 packinghouses. In all these localized points arthropods caught on yellow sticky traps are identified monthly and citrus trees are visually inspected for HLB symptoms . Suspicious HLB-symptomatic leaves were directly printed on paper membranes during the field inspection and in the Canary Islands T. erytreae specimens were directly squashed. Samples were analyzed by real-time PCR using a commercial kit . Neither the analyzed citrus samples nor psyllid species tested positive for ‘Ca. Liberibacter’ HLB-species. T. erytreae was not found in mainland Spain. Implementation of surveys in all Mediterranean countries and legislation for eradication should be a priority for HLB prevention.Citrus greening disease remains a threat in the cooler citrus production areas of southern Africa despite restrictions on the movement of citrus material from infected areas, as well as cultural control measures such as the use of systemic insecticides for vector control, planting of healthy certified trees and the eradication of infected material. The ultimate control strategy would be the use of resistant plant material.A new approach is to utilize embryo rescue of seed from healthy chimera sections on diseased fruit. The embryos were obtained from wide, asymptomatic sections of symptomatic fruit and cultured on Murashige and Tucker medium. Once the clones had developed sufficiently, they were challenged with Candidatus Liberibacter africanus by means of the Triozid insect vector in the laboratory.