Dale, Jon

Huanglongbing (HLB, citrus greening disease) in the major citrus-producing states of the United States is associated with Candidatus Liberibacter asiaticus (CLas), which is vectored by the Asian citrus psyllid (ACP). Surveys were conducted in Texas from 2007 to 2017 to assess the prevalence and titer of CLas in ACPs and citrus trees. ACP and citrus leaf tissue samples were collected from suspect trees in residential areas and commercial groves (orchards) and assayed for CLas by quantitative PCR. CLas detection in ACPs (2011) preceded that of citrus trees (2012) by several months. Annual incidences of CLas-positive ACPs and leaf tissue followed an exponential growth pattern over the survey period, varying from 0.03 to 28.7% in ACPs and 0.6 to 36.5% in citrus trees. There was a significant and positive relationship between the monthly incidences of CLas-positive ACP and leaf tissue samples. The proportion of HLB detection sites also increased with time, reaching 26 and 40% of commercial groves and residential sites, respectively, by 2017. Seasonal variations were observed in the incidences of CLas-positive ACPs and citrus trees such that significantly more CLas-positive ACPs and trees were recorded during the fall and winter of a given year relative to the hot summer. A temporal analysis of the class distribution of cycle threshold values revealed a trend of increased bacterial accumulation in ACPs and trees over time, with the trend more pronounced for the former than the latter host type. These findings provide a comprehensive insight into the ongoing CLas/HLB epidemic in Texas, with potential lessons for California and other citrus-producing areas where the disease is not yet established.

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.