Agronomy and Crop Science

AbstractPathogens which need a vector for their transmission can alter the vectors’ behaviour to favour their spread. We used the electrical penetration graph technique to investigate this hypothesis by using the tomato potato psyllid Bactericera cockerelli infected or not with the plant pathogen Candidatus Liberibacter solanacearum (CLso) on African boxthorn and tomato. Probing was not affected by the host type but there was a significant effect on probing due to the infection status of the psyllid. More psyllids carried out probing activities in the sieve elements when infected with CLso, and more probing activities were observed from CLso-infected psyllids by comparison to the non-infected groups. Specifically, significant increases in salivation, phloem ingestion and number of probes, before and after reaching the sieve elements, were noticed in the infected groups. Furthermore, time elapsed to reach the sieve elements was significantly shortened by 2 h in the infected group. Remaining probing activities in xylem tissues were not different between all psyllid groups. The observed changes in feeding behaviour by pathogen-infected psyllids may well ensure further spread of the pathogen as greater salivation has the potential to increase transmission, highlighting at the same time the important role that crop and non-crop hosts play in disease epidemiology.

‘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’.