Publications

4742

The Asian citrus psyllid, Diaphorina citri Kuwayama, one of the known vectors for citrus greening disease or Huanglongbing (HLB) pathogens, has been present in Texas for over a decade, but the detection of the disease is recent. HLB has been confirmed in only two adjacent commercial citrus groves of grapefruit and sweet orange. A study was conducted to compare the population of Candidatus Liberibacter asiaticus (CLas) cells in different plant parts including peduncle, columella, leaves, seeds, young shoots, flower buds, flowers, and bark of 6-year-old known infected grapefruit and sweet orange trees. The bacterial population was estimated using a previously described grand universal regression equation Y = 13.82 – 0.2866X, where Y is the log of the target copy number and X is the Ct (threshold cycle) of the assay. Except for bark tissue, there was no significant difference in the concentration of CLas cells in other plant parts between the two cultivars. Within the cultivar, the bacterial concentration also varied with the plant part, with peduncle, columella, midrib having significantly higher titer of CLas compared with other plant parts. The obtained results here are in agreement with previous studies conducted on Florida samples, but the consistently lowest bacterial titer recorded in young shoots, leaf blade, and especially leaf margins relative to the midrib has never been previously reported.

Huanglongbing (HLB) disease, associated with ‘Candidatus Liberibacter asiaticus’ (CLas), is primarily spread via infected citrus nursery trees and by infective Asian citrus psyllid, the insect vector. Recently, the Texas Department of Agriculture initiated regulations requiring commercial and retail citrus nurseries in Texas to transition from traditional open-field to enclosed facilities with insect-resistant screens to mitigate the risk of nurseries serving as sources of CLas. Although several nursery production facilities have adopted this regulation, non-enclosed nurseries persist and pose a significant threat to the citrus industry as potential sources of CLas. A systematic survey for HLB was embarked on in a semi-open nursery facility in South Texas in April 2014. Leaf tissue samples taken from 94 trees representing 5% of the total number of potted trees in the nursery were tested for CLas by quantitative and conventional PCR assays. Of 94 trees tested, 3.2% (3 trees) were positive for CLas by both assays. The presence of CLas in the PCR-positive samples was confirmed by multi-locus sequence analyses. The results represent the first report of HLB in a nursery facility in Texas, and underscore the need for more intensive surveillance for HLB in citrus nursery stock as an integral component of HLB mitigation efforts in Texas. Accepted for publication 27 August 2014. Published 15 December 2014.