Studies on the bacterium Candidatus Liberibacter spp., causal agent of citrus huanglongbing (ex-greening): diagnosis, biology and management

Abstract

The citrus industry is one of the most important activities of the agribusiness in Brazil, where the state of São Paulo is the main citrus producer and exporter of Frozen Concentrated Orange Juice (FCOJ). In Brazil, the low yield (around 2 boxes/tree/year) is highly associated to the incidence of pests and diseases (resulting in a substantial influence over the production costs), the production in non-irrigated areas, and the narrow genetic basis that has been used (i.e., only a few varieties grown in most of the orchards). In the past 20 years, many economically important diseases and pests have occurred in Brazil, which includes citrus variegated chlorosis (CVC), leprosis, black spot, alternaria brown spot, sudden death, citrus fruit borer, citrus leaf miner (CLM), scales, etc. Until recently, there has not been reported in the western hemisphere the occurrence of the huanglongbing (ex-greening), an extremely aggressive and damaging citrus disease wherever it occurs in the world. However, in 2004, the causal agent of the disease, the bacterium Candidatus Liberibacter spp., was for the first time found in the state of Sao Paulo in citrus plants that resembled its typical symptoms. Huanglongbing is one of the most destructive and severe diseases of citrus and causes damages in the plant development (chlorosis, leaf drop, and reduction of growth and roots) and fruit (misshapen, seed abortion, drop, and growth reduction). The causal agent is a phloem-limited bacterium, which has not been isolated so far, and is transmitted by the Asian citrus psyllid (Diaphorina citri), an insect easily found in Brazil. Under a multidisciplinary effort that has been made by putting together various research groups in the state of Sao Paulo, this proposal, as a thematic project, seeks to enhance the knowledge on the Huanglongbing pathossystem, which is still completely not understood under the conditions of the state of Sao Paulo. In spite of this, the objectives of this proposal involve to uncover partially the bacterium's genome, develop alternative systems for diagnostics, identify the bacterial diversity, find out how the bacterium multiplies in the citrus and other hosts tissues, study the effects of temperature over the severity and intensity of symptoms and bacterial multiplication as well as to evaluate the acquisition and persistence of the bacterium in the vector insect. Identification of these factors will provide the basis for orderly dissection of the processes underlying pathogenicity and, as a result, ways for the management of the disease and its vector. (AU)