Plant Disease

‘Candidatus Liberibacter asiaticus’ (CLas) is an unculturable, phloem-restricted αProteobacteria, associated with citrus Huanglongbing (HLB), which is one of the most destructive diseases in citrus production worldwide. Here, we present the genome sequences of CLas strains PA19 and PA20 from HLB-affected kinnow trees in Multan, Punjab Province, Pakistan. The CLas genomes of PA19 and PA20 comprise 1,224,156 bp and 1,226,225 bp, respectively, with an average GC content of 36.4%. Both harbored the Type 2 prophage. In this study, we report two CLas genomes from Pakistan, which extends the sequence database of CLas and will contribute to CLas biology and HLB management.

Vegetative grafting is a common method of transmitting and propagating ‘Candidatus Liberibacter asiaticus’, the bacterial species accepted as the causal agent of the citrus disease huanglongbing (HLB). Generally, infected tissue that is grafted to a receptor tree remains in place indefinitely to ensure transmission. In this study, individual HLB-symptomatic leaves were grafted as ‘Ca. L. asiaticus’ inoculum sources to receptor trees of six citrus types and removed after an inoculation period (IP) of 21, 51, or 81 days. The goal was to assess the effect of varying IPs on transmission of bacteria to the receptor tree and on the successful establishment of a new infection. Survival analysis of data from three trials showed a significantly reduced proportion of infected trees with an IP of 21 days compared with IPs of 51 and 81 days but that there was no significant difference in the proportion of infected trees between IPs of 51 and 81 days. In addition, the time to first detection of pathogen DNA in an infected tree was delayed significantly for the 21-day IP when compared with the 51- and 81-day IPs. Survival analysis showed that the probability of infection of sweet orange trees was significantly higher than for trees of five other citrus types throughout the experiment. There was no significant difference between the infection probabilities of these latter five citrus types. The data from this study show that successful infection by grafting is dependent upon the time of exposure to the inoculum, that shorter IPs increase the time needed to establish a systemic infection, and that citrus types vary in their overall susceptibility to infection by ‘Ca. L. asiaticus’.

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.

‘Candidatus Phytoplasma pini’-related strain MDPP, the reference strain of subgroup 16SrXXI-B, is a pathogen associated with witches’ broom disease of Pinus spp. in North America. Here, we report the first draft genome sequence of ‘Ca. Phytoplasma pini’ strain MDPP, which consists of 474,136 bases, with a G + C content of 22.22%. This information will facilitate comparative genomics of gymnosperm-infecting phytoplasmas.

‘Candidatus Liberibacter asiaticus’ (Las) is an unculturable α-proteobacterium associated with citrus huanglongbing (HLB), a devastating disease currently threatening the citrus industry worldwide. Here, we present the genome sequence of Las strain TaiYZ2 from an HLB-affected pomelo tree in Hat Yai district, Songkhla Province, Thailand. The TaiYZ2 genome is composed of 1,230,623 bp with G+C content of 36.4%. This is the first Las genome sequence from Thailand, which will enrich current Las genome resource and facilitate HLB research and management.