Assessments of Temporal Variations in Haplotypes of 'Candidatus Liberibacter solanacearum' and Its Vector, the Potato Psyllid, in Potato Fields and Native Vegetation.

The potato psyllid, Bactericera cockerelli (Sulc) (Hemiptera: Triozidae), had been known for nearly a century to cause psyllid yellows of solanaceous crops. However, it has only been a decade since the insect was discovered to transmit the bacterium 'Candidatus Liberibacter solanacearum' (Lso), which putatively causes potato zebra chip disease. This project was initiated to quantify temporal incidences of haplotypes of the psyllid (Central, Southwestern, and Western) and Lso (A, B) in potato fields and in native vegetation. Psyllids were collected from native vegetation in Texas (2011-2014), and from potato fields in Texas and New Mexico (2014-2017). Psyllids were tested for Lso and haplotypes of both psyllid and Lso. In Texas, the Central psyllid haplotype was overwhelmingly dominant both in potato fields and in native vegetation regardless of location and time of collection. However, in New Mexico potato fields, although the Southwestern haplotype was overall dominant, the ratios of individual haplotypes varied among years and within a season. The Southwestern psyllid haplotype was greater in incidence than the Central early but declined later in the season in each of the 4 yr, while the Central haplotype was low in incidence early but increased over time. Lso was detected in all three psyllid haplotypes representing the first report in Southwestern psyllid haplotype. In Texas, Lso haplotype A was more frequently detected than B, but in New Mexico the incidence of positive psyllids was not high enough to make definitive conclusions regarding predominant Lso haplotype.

Postharvest Development of 'Candidatus Liberibacter solanacearum' in Late-Season Infected Potato Tubers under Commercial Storage Conditions.

Zebra chip (ZC) disease of potato is associated with the putative pathogen 'Candidatus Liberibacter solanacearum', which is transmitted by the potato psyllid Bactericera cockerelli (Hem., Triozidae). The present study was initiated to investigate 'Ca. L. solanacearum' development during and following typical commercial storage practices. Using bacteriliferous psyllids, Russet Norkotah potato tubers were infested in field cages 14, 10, and 4 days before harvest. Changes in 'Ca. L. solanacearum' detection rate, 'Ca. L. solanacearum' titer, and concentrations of phenolic compounds were documented throughout storage. 'Ca. L. solanacearum' titer continued to increase during storage. Although significant increases in the frequency of 'Ca. L. solanacearum' detection were observed in all infestation treatments, the impact of 'Ca. L. solanacearum' infection on tuber quality remained comparatively low in plants infected 4 days before harvest, because the majority of the tubers remained asymptomatic. Minimizing storage and retail chain movement durations would help to limit 'Ca. L. solanacearum' impact on tuber quality in tubers infected 14 and 10 days before harvest. This study also demonstrated that 'Ca. L. solanacearum' can relocate from a newly infected leaf to a tuber in as little as 4 days. Psyllid management is recommended until at least 4 days before green harvest, when psyllid pressure is high in fields in which tubers are destined for commercial storage.

Variations in Zebra Chip Disease Expression and Tuber Biochemistry in Response to Vector Density.

Zebra chip (ZC) disease, caused by 'Candidatus Liberibacter solanacearum', which is transmitted by the potato psyllid, has negatively affected potato production in the United States for over a decade. The present study was conducted to evaluate the affect of the number of bacteriliferous psyllids on 'Ca. L. solanacearum' titer, levels of amino acids, carbohydrates, phenolics, and, subsequently, symptom severity in potato tubers. 'Red La Soda' and 'Russet Norkotah' potato were planted in the field and later inoculated with 'Ca. L. solanacearum' using 5, 10, and 30 bacteriliferous potato psyllids. In both cultivars, the increase in the number of psyllids resulted in elevated 'Ca. L. solanacearum' titer and symptom severity. In the cases of amino acids and reducing sugars, responses to vector density appeared to be cultivar specific. Overall, phenolic compounds showed a consistent increase following infection, a response that, in most cases, was positively correlated with the number of infective psyllids. Results quantified the impact of the number of infective psyllids on 'Ca. L. solanacearum' titer and biochemical content of infected tubers previously shown to be correlated with the degree of symptom expression. Managing vector numbers is currently the only effective approach for minimizing losses to ZC in grower's fields. However, our findings on physiological responses to vector density suggest that, in combination with chemical control, development of more tolerant or resistant potato genotypes, with emphasis on interrupting pathways that are associated with increased phenolic activity levels, may lead to more sustainable management of ZC in the future.

A decision-support tool to predict spray deposition of insecticides in commercial potato fields and its implications for their performance.

Approximately US $1.3 billion is spent each year on insecticide applications in major row crops. Despite this significant economic importance, there are currently no widely established decision-support tools available to assess suitability of spray application conditions or of the predicted quality or performance of a given commercial insecticide applications. We conducted a field study, involving 14 commercial spray applications with either fixed wing airplane (N=8) or ground rig (N=6), and we used environmental variables as regression fits to obtained spray deposition (coverage in percentage). We showed that (1) ground rig applications provided higher spray deposition than aerial applications, (2) spray deposition was lowest in the bottom portion of the canopy, (3) increase in plant height reduced spray deposition, (4) wind speed increased spray deposition, and (5) higher ambient temperatures and dew point increased spray deposition. Potato psyllid, Bactericera cockerelli (Sulc) (Hemiptera: Triozidae), mortality increased asymptotically to approximately 60% in response to abamectin spray depositions exceeding around 20%, whereas mortality of psyllid adults reached an asymptotic response approximately 40% when lambda-cyhalothrin/thiamethoxam spray deposition exceeded 30%. A spray deposition support tool was developed (http://pilcc.tamu.edu/) that may be used to make decisions regarding (1) when is the best time of day to conduct spray applications and (2) selecting which insecticide to spray based on expected spray deposition. The main conclusion from this analysis is that optimization of insecticide spray deposition should be considered a fundamental pillar of successful integrated pest management programs to increase efficiency of sprays (and therefore reduce production costs) and to reduce risk of resistance development in target pest populations.