Agronomy and Crop Science

The phloem-limited ‘Candidatus Liberibacter asiaticus’ (Las) causes huanglongbing, a destructive citrus disease. Graft-inoculated potted plants were used to assess Las speed of movement in phloem in the greenhouse, and the impacts of temperature on plant colonization in growth-chamber experiments. For assessment of Las speed, plants were inoculated at the main stem and assessed over time by quantitative PCR (qPCR) or symptoms at various distances from the inoculum. For colonization, the plants were inoculated in one of two opposite top branches, maintained at from 8 to 20°C, from 18 to 30°C, or from 24 to 38°C daily range, and assessed by qPCR of samples taken from noninoculated shoots. For all experiments, frequencies of Las-positive sites were submitted to analysis of variance and binomial generalized linear model and logistic regression analyses. Probabilities of detecting Las in greenhouse plants were functions of time and distance from the inoculation site, which resulted in 2.9 and 3.8 cm day−1 average speed of movement. In growth chambers, the temperature impacted plant colonization by Las, new shoot emission, and symptom expression. After a 7-month exposure time, Las was absent in all new shoots in the cooler environment (average three per plant), and present in 70% at the milder environment (six shoots, severe symptoms) and 25% in the warmer environment (eight shoots, no visible symptoms). Temperature of 25.7°C was the optimum condition for plant colonization. This explains the higher impact and incidence of huanglongbing disease during the winter months or regions of milder climates in Brazil.

The potato psyllid Bactericera cockerelli (Šulc) (Hemiptera: Triozidae) is a pest of solanaceous crops (order Solanales), including potato (Solanum tuberosum L.) and tomato (S. lycopersicum L.). Feeding by high populations of nymphs causes psyllid yellows while adults and nymphs are vectors of the plant pathogen ‘Candidatus Liberibacter solanacearum’. Foliar symptoms that were consistent with either ‘Ca. L. solanacearum’ infection or psyllid yellows were observed in 2019 on tomatillo (Physalis ixocarpa Brot.; family Solanaceae) grown within an experimental plot located near Saltillo, Mexico. This study had three primary objectives: 9i) determine whether the foliar symptoms observed on tomatillo were associated with ‘Ca. L. solanacearum’ infection, (ii) identify the haplotypes of ‘Ca. L. solanacearum’ and potato psyllids present in the symptomatic plot, and (iii) use gut content analysis to infer the plant sources of ‘Ca. L. solanacearum’-infected psyllids. Results confirmed that 71% of symptomatic plants and 71% of psyllids collected from the plants were infected with ‘Ca. L. solanacearum’. The detection of ‘Ca. L. solanacearum’ in plants and psyllids and the lack of nymphal populations associated with psyllid yellows strongly suggests that the observed foliar symptoms were caused by ‘Ca. L. solanacearum’ infection. All infected plants and insects harbored the more virulent ‘Ca. L. solanacearum’ haplotype B but one psyllid was also coinfected with haplotype A. The potato psyllids were predominantly of the central haplotype but one psyllid was identified as the western haplotype. Molecular gut content analysis of psyllids confirmed the movement of psyllids between noncrop habitats and tomatillo and indicated that ‘Ca. L. solanacearum’ infection of psyllids was associated with increased plant diversity in their diet.

Specific scopeThis Standard describes a diagnostic protocol for ‘Candidatus Liberibacter africanus’, ‘Candidatus Liberibacter americanus’ and ‘Candidatus Liberibacter asiaticus’.1This Standard should be used in conjunction with PM 7/76 (5) Use of EPPO diagnostic protocols.Specific approval and amendmentFirst approved in 2014–09. Revised in 2021–04.

AbstractBACKGROUNDDiaphorina citri is a vector of ‘Candidatus Liberibacter asiaticus’ (CLas), which is associated with citrus huanglongbing (HLB). In this study, the adaptability and CLas titres of D. citri adults on three weed species, namely, Ageratum conyzoides, Solanum nigrum and Praxelis clematidea, which are widely distributed in citrus orchards in China, were determined.RESULTSD. citri selected S. nigrum preferentially over the other weed species. The longest survival times of D. citri adults on A. conyzoides, S. nigrum and P. clematidea were 48, 41 and 11 days, respectively. On S. nigrum, the survivorship of females was significantly higher than that of males. The CLas titres of D. citri adults feeding on the three weeds for 1 week increased significantly during autumn but did not change significantly during spring. Conversely, the titres of adults on citrus increased significantly during spring but did not change significantly during autumn.CONCLUSIOND. citri exhibited different adaptabilities to the three weed species, but the CLas titres of D. citri adults did not decrease on any of the weeds. These results suggested that the three weed species could help dispersing of D. citri and transmission of CLas while ideal host conditions were scarce or absent. © 2021 Society of Chemical Industry.