Plant Disease

Zebra chip (ZC) disease, a serious threat to the potato industry, is caused by the bacterium ‘Candidatus Liberibacter solanacearum’ (Lso). Five haplotypes (hapA to hapE) of this pathogen have been described so far in different crops, with only hapA and hapB being associated with ZC in potato. Both haplotypes are vectored and transmitted to a variety of solanaceaeous plants by the tomato/potato psyllid, Bactericera cockerelli (Šulc). Psyllids are native to North America, and four haplotypes have been identified and named based on their predominant geographic association: Northwestern, Central, Western, and Southwestern. Although all psyllid haplotypes have been found in southern Idaho potato fields, data on relative haplotype abundances and dynamic changes in the fields over time have not previously been reported. Here, psyllid samples collected in Idaho potato fields from 2012 to 2015 were used to clarify spatial and temporal patterns in distribution and abundance of psyllid and Lso haplotypes. A shift from hapA toward hapB population of Lso was revealed during these four seasons, indicating possible evolution of Lso in Idaho fields. Although we confirmed that Western psyllids were the most abundant by far during the four seasons of observation, we also observed changes in abundance of other haplotypes, including increased diversity of psyllid haplotypes during 2015. Seasonal changes observed for the Northwestern and Central haplotypes could potentially be linked to psyllid migration and/or habitat changes. South-central Idaho exhibited more diversity in psyllid haplotypes than southwestern Idaho.

The major citrus area of Brazil occupies near 450,000 ha between the Triângulo Mineiro (TM) region of Minas Gerais State and the south of São Paulo State (SPS). Significant climatic variation occurs between regions which could affect huanglongbing (HLB) progress, which is lower in TM. To investigate this possibility, young sweet orange shoots were sampled periodically over 2 years to determine ‘Candidatus Liberibacter asiaticus’ titers in naturally infected trees in orchards in Analândia, central SPS, and Frutal and Comendador Gomes, within TM. Data-loggers recorded local temperature and relative humidity hourly. In the laboratory, five ‘Ca. L. asiaticus’-free Diaphorina citri adults were placed on each sampled shoot for 48 h to feed and acquire the pathogen. Shoots and insects were individually analyzed by quantitative polymerase chain reaction to determine ‘Ca. L. asiaticus’ titers. The incidence of ‘Ca. L. asiaticus’-positive shoots, ‘Ca. L. asiaticus’ titers, and acquisition rates were lower for shoots from Comendador Gomes than those from Frutal or Analândia. Stronger association was observed between ‘Ca. L. asiaticus’ titers and the number of hours below 15°C (h < 15°C) or above 30°C (h > 30°C), and cumulative rainfall registered during the 15 days prior to sampling of shoots on each occasion. ‘Ca. L. asiaticus’ titers associated positively with h < 15°C and rainfall and negatively with h > 30°C. The slower spread and lower incidence of HLB in TM may be related to lower incidences of ‘Ca. L. asiaticus’-positive young shoots and lower titers of ‘Ca. L. asiaticus’ in the same shoots as a consequence of the warmer and drier conditions.

Huanglongbing (HLB) is a difficult-to-control and highly destructive citrus disease that, in Brazil, is associated mainly with the bacterium ‘Candidatus Liberibacter asiaticus’ transmitted by the psyllid Diaphorina citri. The aim of this study was to improve our understanding of the ‘Ca. L. asiaticus’ infection process by exposing excised, fully expanded, immature citrus leaves in 50-ml Falcon tubes to one, four, or eight adults from a ‘Ca. L. asiaticus’-exposed colony for 1-, 3-, 7-, or 15-day periods for access to inoculation (IAP). The leaves were incubated at 26°C for 1, 3, 7, 15, and 21 days (incubation period [IP]). Infection frequencies and ‘Ca. L. asiaticus’ titers were assessed by quantitative polymerase chain reaction (qPCR). ‘Ca. L. asiaticus’ infection was a function of leaf age, number of insects, IAP, and IP. In general, higher infection rates were observed on younger leaves inoculated with higher numbers of insects and after longer IAP and IP. The immature excised leaf method allowed determination of 3 to 7 days as the range of time required by ‘Ca. L. asiaticus’ to reach qPCR detectable levels. Even though leaf survival could be prolonged by the maintenance of a branch segment at the base of the leaf petiole, leaf degradation, visible after about 15 days IP, did not allow observation of the entire infection process which, in the intact plant, culminates with the appearance of the blotch mottling symptom on leaf blades.