Huanglongbing (HLB) incidence on 2-3 years old tangerine trees (Citrus reticulata) grown from disease-free nursery stock

Abstract

Huanglongbing (HLB) previously referred to as greening disease is a bacterial disease of citrus trees, and a major problem worldwide. HLB affects citrus trees by blocking the phloem or the vascular system of the tree, limiting its ability to uptake nutrients. Asian citrus psyllidDiaphorina citri (Hemiptera: Psyllidae) is the vector, transmitting Candidatus Liberibacter asiaticus (Las). In this study in 2012-2014, occurrence of HLB disease after infection was monitored in an experimental planting of 589 tangerine trees (Citrus reticulata) grown from disease-free seedlings in a nursery in Chiang Mai, Thailand.  An early symptom of HLB on tangerine is the yellowing of leaves on an individual limb or in one sector of a tree's canopy. Field trees can be identified with suspected HLB infection by their foliar but verification requires DNA detection methods. Las can be detected in DNA extracted from infected plants and psyllids by polymerase chain reaction (PCR). An efficient method for DNA extraction from infected plants and psyllids was studied.  Polymerase chain reaction (PCR) was performed by using Las606/LSS primer to detect symptomatic leaves. Specific primers, forward primer Las606 (5’- GGA GAG GTG   AGT GGA ATT CCG A-3’) and reverseprimer LSS (5’- ACC CAA CAT CTA GGT AAA AAC C -3’) were used for amplification of the16S rDNA of Candidatus Liberibacter asiaticus (Las), producing specific bands of 500 bp. The study area showed an increasing  percentage of disease incidence,  from 0.84% in 2012 to 4.41% in 2013 and 11.54% in 2014. It appears that there is a high incidence of HLB-infected trees at the edges of the plantation. One of potential HLB pathways is infected Asian citrus psyllids from natural movement. The Asian citrus psyllid vector of HLB has a wide host range, can achieve high populations at citrus vegetative flush, can be spread over long distances, and its control demands both continuous inspection and regular insecticide applications.