Identification of QTL Associated with Citrus Tolerance to HLB Caused By ‘Candidatus Liberibacter asiaticus’

Huanglongbing (HLB or greening) caused by the bacterium Candidatus Liberibacter is currently one of the most serious diseases of citrus and has caused substantial economic losses. So far there is no source of genetic resistance to HLB in Citrus or its relatives. However several studies report the group of Poncirus trifoliata and some of its hybrids, as more tolerant to the disease. We have an F₁ population comprising of 274 individuals of an interspecies cross between the Citrus sunki and Poncirus trifoliata. These hybrids are established in field about 7 years in an area with high rates of infection and with no HLB management. Leaves were collected for DNA extraction from leaf midrib for diagnosis. qPCR reactions were carried out using TaqMan® PCR system with primers-probes previously described (16S rDNA) and GAPDH as internal control. As results, bacteria was not able to multiply in all of the plants, a wide range of responses were observed among different hybrids. After that, this survey was used as phenotypical data and incorporated in highly saturated maps constructed with Genotyping by Sequencing (GBS) markers. Analysis of quantitative trait loci (QTL) associated with resistance to HLB was performed from linkage maps of the parents with 11 and 10 linkage groups for Sunki and Trifoliata, respectively, with a total of 1,200 DArT-GBS markers. The wide range of responses observed among hybrids and the detection of several QTL in the linkage groups of Poncirus with moderate effects support the hypothesis that resistance to HLB is polygenic.