citrus greening

Citrus greening or Huanglongbing (HLB) is one of the most severe diseases of citrus worldwide and no cure is available at present. Currently molecular techniques based on polymerase chain reaction (PCR) are mainly used for the identification of HLB disease. The aim of this study was to identify the causal pathogen of citrus greening disease in Bangladesh. A total of eight mandarin orange leaf samples from three citrus growing areas viz. Moulvibazar, Sylhet and Khagrachari were collected on the basis of visual investigation demonstrating blotchy mottle symptoms. The genomic DNA was extracted from midribs of each leaf sample and subjected to the PCR. Species specific primers A2/J5 was used to amplify a partial sequence of β-operon (rplKAJL-rpoBC operon) of ribosomal protein gene. An amplicon size of 703 bp specific to the Candidatus Liberibacter asiaticus confirmed its presence. BLAST homology search with the sequenced data showed 99% identity with the sequence database of Candidatus Liberibacter asiaticus deposited in GenBank. These results confirmed the prevalence of Candidatus Liberibacter asiaticus causing citrus greening in Bangladesh.

Huanglongbing (HLB) or citrus greening is the most economically devastating disease of citrus in the world. HLB is a vector-borne disease and transmitted by Asian Citrus psyllid (ACP). HLB is now a serious threat to the cultivation and expansion of Sweet orange and Mandarin in Bangladesh. As no suitable cure is available against the disease, inducing plant immunity by chemical inducers or nutrient management and intercropping could be an effective way to combat this challenge. In this study, two inducers viz., Bion (Acibenzolar S-methyl) and Bactroban (Bismerthizol), nutrients formulations SICOGREEN® (soil application) and foliar spray, intercropping with guava, spraying guava leaf extract (10%), foliar spray with insect growth regulators (IGR) such as Heron (Lufenuron), insecticides such as Neonicotinoids/ Imidachloropid + Thiomethoxam and foliar spray of Beauveria bassiana (Commercial formulation) showed comparatively better performance as compared to untreated control considering both HLB incidence and severity in both locations (Haluaghat and Bhaluka) of Sweet orange orchards. All these treatments reduced HLB incidence by 57.5 to 89.44% and HLB severity by 54.16 to 80.35% in Sweet orange considering both Haluaghat and Bhaluka orchards. The results revealed that Bion (Acibenzolar S-methyl), nutrients formulations SICOGREEN® (soil and foliar application), intercropping with guava, spraying guava leaf extract, foliar spray of insecticides can be integrated to reduce HLB incidence and severity in Sweet orange. Some of these treatments have also some positive effects on plant growth and yield parameters of Sweet orange as compared to control. These results comprehensively suggest that chemical inducers and nutrient management seem a better alternative to control HLB aimed to increase tree lifespan and productivity. Keywords Disease control Integrated approach Huanglongbing Sweet orange Citation of this article: Islam, M.R., Islam, M.N., Alam, M.Z., Hossain, M.M., Hussen, M.A.M. and Tipu, M.M.H. (2020). Chemical inducers, nutrient management, guava intercropping and insecticides can reduce huanglongbing incidence and severity in Sweet orange.

The second largest fruit crop of the world, citrus is a high prospect sector in Nepalese hills. Citrus greening (Huanglongbing) is the most complex, challenging, incalcitrant and devastating disease that has threatened the citrus industry worldwide. Citrus Decline in Nepal is claimed to be because of HLB coupled with other diseases and grove management practices. This disease is spread by a sucking insect, the Asian Citrus Psyllid (ACP); by controlling its population the spread of HLB to new groves could be prevented. This review article comprises the best method for rapid detection of HLB and chemical, biological and cultural management practices for controlling ACP to prevent the spread of HLB in Nepal. Iodine based test can be used to screen the samples infested with HLB which could be confirmed after molecular HLB testing i.e. PCR. Chemically, ACP could be best managed by intensive chemical spray during the critical flushing periods and winter. Biologically, ACP could be best managed by the use of parasitoids, predators and pathogen that consumes and destroy ACP nymphs/or adults. Culturally, the spread of ACP could be best managed by protective cultivation, reflective mulch and growing nurseries to produce saplings at high altitudes in Nepal for preventingthe spread of HLBin new groves.

Citrus huanglongbing (greening) is one of the important diseases of citrus in Asia (Bové, 2006). The symptoms include blotchy chlorosis and/or mottling of the leaves, stunted growth, poor root growth, small, green, and malformed fruits and decline of the trees. The disease is caused by the phloem-limited unculturable fastidious bacteria Candida-tus Liberibacter asiaticus (CLas), Ca. L. africanus (CLaf), and Ca. L. americanus (CLam) (Bové, 2006; Jagoueix et al., 1994). Among them CLas is the most widespread (e.g., Asia, Brazil and North America). In recent years the presence of huanglongbing was suspected in different regions of Bangladesh. To establish whether this is the case, in July, 2016 symptomatic plant samples of Citrus sinensis were collected from four citrus-growing areas of the country: Jaintapur (Sylhet), Akbarpur (Moulvibazar), Dighinala (Khagrachari) and Hathazari (Chittagong). The samples were analyzed by polymerase PCR using the ‘CLas’ specific primers A2 (5’-TATA A AGGTTGACCTTTCGAGTTT-3’)  and  J5  (5’-ACA A A AGCAGA A ATAGCACGA ACA A-3’) which amplify a partial sequence of the β-operon (rplKAJL-rpoBC operon) of ribosomal protein genes (Hocquellet et al., 1999). A product 703 bp in size specific to Ca. L. asiaticus was amplified, sequenced and deposited in GenBank (accession No. KX826950). The nucleotide sequence of the recovered CLas shared 100% sequence identity with other Ca. L. asiaticus strains from NCBI database (e.g. KC477384, KT164844, KC133065). To our knowledge this is the first molecular-based detection of Candidatus Liberibacter asiaticus infecting Citrus sinensis in Bangladesh.

The African citrus psyllid, Trioza erytreae, vectors citrus greening or huanglongbing (HLB) disease. The psyllid has been reported from mainland Europe, where it is rapidly spreading from the northwest to the southwest of the Iberian Peninsula. In order to reduce its spread and population levels, a classical biological control program with the parasitoid Tamarixia dryi is under development in Spain. We evaluated the host specificity of T. dryi using 11 non-target psyllid (NTP) species, including five species of the genus Trioza. The psyllids were selected based on phylogenetic and ecological criteria. Tamarixia dryi exhibited a high host specificity. Females did not parasitize any of the 11 NTPs tested, except for one nymph of a gall-forming Trioza species closely related to Trioza montanetana. Tamarixia dryi only laid one egg on a nymph when it was removed from the gall on Convolvulus canariensis and exposed directly to the parasitoid. However, the immature parasitoid died before emerging. We further confirmed that T. dryi did not parasitize a representative triozid species, Trioza laurisilvae, of the endemic Canarian fauna after long time exposure. Our results demonstrate that T. dryi is a highly specific parasitoid and its introduction, release and establishment in Europe within the classical biological control program of T. erytreae should not affect other psyllid species. Therefore, no significant environmental impact is expected.