First Report of Zebra Chip Disease and "Candidatus Liberibacter solanacearum" on Potatoes in Idaho

A comprehensive review of zebra chip disease in potato and its management through breeding for resistance/tolerance to 'Candidatus Liberibacter solanacearum' and its insect vector

Zebra chip disease (ZC), associated with the plant pathogenic bacterium 'Candidatus Liberibacter solanacearum' (psyllaurous) (CLso), is a major threat to global potato production. In addition to yield loss, CLso infection causes discoloration in the tubers, rendering them unmarketable. CLso is transmitted by the potato psyllid, Bactericera cockerelli (Šulc) (Hemiptera: Triozidae). ZC is managed by prophylactic insecticide applications to control the vector, which is costly and carries environmental and human health risks. Given the expense, difficulty, and unsustainability of managing vector-borne diseases with insecticides, identifying sources of resistance to CLso and developing varieties that are resistant or tolerant to CLso and/or potato psyllids has become a major goal of breeding efforts. These efforts include field and laboratory evaluations of noncultivated germplasm and cultivars, studies of tubers in cold storage, detailed quantifications of biochemical responses to infection with CLso, possible mechanisms underlying insect resistance, and traditional examination of potato quality following infections. This review provides a brief history of ZC and potato psyllid, a summary of currently available tools to manage ZC, and a comprehensive review of breeding efforts for ZC and potato psyllid management within the greater context of Integrated Pest Management (IPM) strategies. © 2022 Society of Chemical Industry. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.

Haplotyping the Potato Psyllid (Hemiptera: Triozidae) and the Associated Pathogenic Bacterium 'Candidatus Liberibacter solanacearum' in Non-crop Alternative Hosts in Southern Idaho

Zebra chip, is a potato disease associated with the bacterium 'Candidatus Liberibacter solanacearum' (Lso) and vectored by the potato psyllid, Bactericera cockerelli Šulc. Potato psyllids are native to North America, where four haplotypes have been described. They are able to colonize a wide range of solanaceous species, crops, and weeds. The epidemiology of zebra chip disease is still poorly understood and might involve the different haplotypes of psyllids as well as two haplotypes of Lso. As several perennial weeds have been recognized as potential host for potato psyllids and Lso, a yearly monitoring of several patches of bittersweet nightshade (Solanum dulcamara) and field bindweed (Convolvulus arvensis) located in the potato-growing region of southern Idaho was conducted from 2013 to 2017, to gain insight into psyllid dynamics in non-potato hosts and Lso presence in the fields. Potato psyllids caught on each host were individually tested for Lso, and a subset were haplotyped based on the CO1 gene, along with the haplotyping of Lso in positive samples. On bittersweet nightshade, the Northwestern haplotype was numerically dominant, with around 2.7% of psyllids found to be carrying either Lso haplotype A or B, suggesting a limited role in zebra chip persistence, which has infected Idaho fields at a low occurrence since the 2012 outbreak. Field bindweed was found to be a transient, non-overwintering host for potato psyllid of Northwestern, Western and Central haplotypes late in the season, suggesting minor, if any, role in persistence of Lso and field infestation by potato psyllids.

Growth and Yield Performance of Solanum tuberosum Grown from Seed Potatoes Infected with 'Candidatus Liberibacter solanacearum' Haplotypes A and B

Zebra chip (ZC) disease of potato (Solanum tuberosum) is associated with infection by 'Candidatus Liberibacter solanacearum' (Lso). Two haplotypes of Lso-A and B-occur in the United States. Lso haplotype B is more virulent than haplotype A, causing greater disease incidence in tubers, more severe symptoms, and greater loss in tuber yield. This study assessed whether tubers from infected plants generate new infected plants the following year. The effects of both Lso haplotypes A and B on tuber resprout were examined on five potato cultivars. When compared with noninfected tubers, overall plant emergence rate from Lso A- or B-infected tubers was lower, plants emerged slower, and plants generated lower daughter tuber yields in weight and quantity. Plants generally emerged poorly from Lso B-infected tubers and produced lower daughter tuber yields than Lso A-infected tubers. Regardless of Lso treatment, all daughter tubers were asymptomatic, and only 0.3% tested positive for Lso in experiments conducted over 2 years. This suggests that plants generated from Lso A- and Lso B-infected seed potatoes with severe ZC symptoms are likely not a significant source of Lso in potato fields.

Potato Psyllid (Hemiptera: Triozidae) Behavior on Three Potato Genotypes With Tolerance to 'Candidatus Liberibacter solanacearum'

The potato/tomato psyllid Bactericera cockerelli (Šulc) transmits 'Candidatus Liberibacter solanacearum' (Lso) (also known as 'Candidatus Liberibacter psyllaurous'), the bacterium associated with zebra chip disease (ZC) in potato. When disease incidence is high, ZC causes large economic losses through reductions in potato yield and tuber quality. No commercial potato variety has been found totally resistant to the pathogen. We evaluated host acceptance behaviors using no-choice assays on three breeding clones derived from Solanum chacoense Bitter with putative tolerance to Lso and/or ZC as part of an effort to determine whether the disease tolerance observed in those breeding clones was related to effects on psyllid settling behavior. We also counted the number of eggs laid and nymphs hatched on the different genotypes to observe any differences in reproduction. The potato variety 'Russet Burbank' was used as a susceptible control. Probing frequency and female walking duration were greater on Russet Burbank than the other genotypes. Oviposition did not differ among genotypes. However, female psyllids on two of the Lso-tolerant genotypes displayed reduced fertility 18-24 d after confinement with a male, relative to females on Russet Burbank. These results suggest that although the germplasms display minor abiotic activity on psyllid fertility, tolerance to Lso may be more strongly linked with plant tolerance to the pathogen rather than effects on host acceptance behaviors.

Interactions Between Solanaceous Crops and 'Candidatus Liberibacter solanacearum' Haplotypes in Relation to Infection and Psyllid Survival on the Hosts

'Candidatus Liberibacter solanacearum' (Lso), transmitted by the potato psyllid (Bactericera cockerelli), is the putative causal agent of potato zebra chip disease. The bacterial pathogen infects a wide range of solanaceous plants (both wild and cultivated species), among which are peppers, potatoes, and tomatoes. Currently there are two commonly detected, genetically distinct haplotypes of Lso (A and B) identified from potatoes in the United States. To determine whether there are interactions between Lso haplotypes and different solanaceous hosts, experiments were conducted in the greenhouse in which pepper, potato, and tomato plants were infested with psyllids carrying Lso A, B, or an A and B mix (AB) or with psyllids free of Lso. Host plants were grown in pots in cages on the greenhouse benches and infested with six psyllids per plant. In addition, eight pepper cultivars were similarly infested for deeper understanding of host-haplotype interactions. Approximately 7 weeks after infestation, adult psyllids in each cage were counted to determine the impact of Lso haplotype-host interactions on psyllid survival and plants were sampled and tested molecularly for Lso. Individual psyllids carrying haplotypes B or AB and those free of Lso copiously reproduced on all three hosts, and leaf tissue from each plant tested positive for the respective Lso except those infested with Lso-negative psyllids. However, psyllids carrying Lso A did not survive on peppers but survived and abundantly reproduced on potatoes and tomatoes. In addition, samples from peppers infested with psyllids carrying Lso A tested negative for Lso. However, peppers infested with individual psyllids carrying Lso AB tested positive for Lso A, indicating that the presence of B may be required for infection by Lso A and psyllid survival on peppers. The different pepper cultivars infested with psyllids carrying Lso A showed similar results to the haplotype-host interaction tests, suggesting that cultivar may not be a factor in Lso A-pepper host interactions. Results from these studies suggest that Lso A may affect host selection by psyllids either for nutrition or laying of eggs. Mechanisms involved in preventing psyllid reproduction on peppers, once identified, will have significant implications for potential psyllid management.

Seasonal Occurrence of Potato Psyllid (Bactericera Cockerelli) and Risk of Zebra Chip Pathogen (Candidatus Liberibacter Solanacearum) in Northwestern New Mexico

Potato psyllid (Bactericera cockerelli) is one of the most important pests in potatoes (Solanum tuberosum L.) due to its feeding behavior and the transmission of a bacterium (Candidatus Liberibacter solanacearum) that causes zebra chip disease, altering the quality of the potato tuber and the fried potato chip or french fry. This pest is thus a threat to the chip potato industry and often requires preventive measures including the use of costly insecticides. The objectives of this research were to monitor the variation in B. cockerelli adult abundance and to evaluate the risk of zebra chip disease in northwestern New Mexico, USA. Yellow sticky traps were used to collect the pest at the Agricultural Experiment Station at Farmington, NM and in nearby commercial fields at the Navajo Agricultural Products Industry (NAPI) and Navajo Mesa Farms during the 2017–2019 period. The collected adult pests were analyzed at Texas A & M University for the presence of Candidatus L. solanacearum (Lso). The results showed field infestation by B. cockerelli in early June and that the population peaked during the second half of July and decreased as the potato growing season progressed. However, a second less important peak of the pest was revealed around mid- to late-August, depending on the growing season and field. While the B. cockerelli population increased linearly with average air temperature, it showed strong third order polynomial relationships with the accumulated thermal units and the Julian days. The test of B. cockerelli for the Lso infection revealed a low incidence of the pathogen varying from 0.22% to 6.25% and the infected adult B. cockerelli were collected during the population peak period. The results of this study may be helpful to potato growers in pest management decision-making and control. However, more study is needed to evaluate zebra chip disease in terms of its prevention and economic impact, and to develop economic thresholds and pest management programs for northwestern New Mexico and neighboring regions.

Prevalence of 'Candidatus Liberibacter solanacearum' Haplotypes in Potato Tubers and Psyllid Vectors in Idaho From 2012 to 2018

'Candidatus Liberibacter solanacearum' (Lso) is an uncultured, phloem-associated bacterium causing a severe tuber disease in potato called zebra chip (ZC). Seven haplotypes of Lso have been described in different hosts, with haplotypes A and B found associated with infections in potato and tomato. In the field, Lso is transmitted by the potato psyllid (Bactericera cockerelli), and between 2011 and 2015, a significant change in Lso haplotype prevalence was previously reported in Idaho: from exclusively A haplotype found in tested psyllids in 2012 to mainly B haplotype found in collected psyllids in 2015. However, prevalence of Lso haplotypes in Idaho was not analyzed in potato tubers exhibiting symptoms of ZC. To fill in this knowledge gap, prevalence of Lso haplotypes was investigated in potato tubers harvested in southern Idaho between 2012 and 2018, and it was found to change from exclusively A haplotype in the 2012 season to an almost equal A and B haplotype distribution during the 2016 season. During the same period, haplotype distribution of Lso in psyllid vectors collected using yellow sticky traps also changed, but in psyllids, the shift from A haplotype of Lso to B haplotype was complete, with no A haplotype detected in 2016 to 2018. The changes in the haplotype prevalence of the Lso circulating in potato fields in southern Idaho may be, among other factors, responsible for a decrease in the ZC incidence in Idaho potato fields between an outbreak of the disease in 2012 and a very low level of ZC afterward.

Associations of the Potato Psyllid and "Candidatus Liberibacter solanacearum" in Idaho with the Noncrop Host Plants Bittersweet Nightshade and Field Bindweed

Zebra chip disease (ZC) in potato (Solanum tuberosum L. [Polemoniales: Solanaceae]) can produce unmarketable tubers with striped necrotic patterns. ZC is associated with the bacterium "Candidatus Liberibacter solanacearum" (Lso), which is transmitted by the potato psyllid, Bactericera cockerelli (Šulc) (Hemiptera: Triozidae). Potato psyllids are associated with numerous noncrop host plants, especially from the Solanaceae and Convolvulaceae; however, the contribution and importance of these hosts to ZC epidemiology in potato is poorly understood. To clarify seasonal phenologies on two such hosts, we sampled potato psyllids from bittersweet nightshade, Solanum dulcamara L. (Polemoniales: Solanaceae), and field bindweed, Convolvulus arvensis L. (Polemoniales: Convolvulaceae), over 2013-2017 and 2014-2016, respectively. Adult psyllids were sampled using yellow sticky traps, vacuum samples, and beat sheets. Each psyllid was tested for the presence of Lso by polymerase chain reaction. Psyllids often were abundant on bittersweet nightshade during May to November, with low numbers observed over each winter. Vacuum samples often captured more psyllids than other methods. Lso incidence was low except during 2016 when vacuum samples showed 23% incidence. Potato psyllids regularly overwinter on bittersweet nightshade in Idaho; however, differences in psyllid populations and Lso incidence from those found on potato suggest that this host plant may only partly contribute to infestations in potato. Observations of psyllids on field bindweed suggest only transient visits to this plant around potato harvest, with no evidence of overwintering and no Lso detected. Further work is needed to clarify how potato psyllid use of other noncrop hosts is related to their abundance in Idaho potato fields.

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.

Examining the Potential Role of Foliar Chemistry in Imparting Potato Germplasm Tolerance to Potato Psyllid, Green Peach Aphid, and Zebra Chip Disease

Long-term, sustainable management of zebra chip disease of potato, caused by 'Candidatus Liberibacter solanacearum' (Lso) and vectored by potato psyllids (Bactericera cockerelli Sulc [Hemiptera: Triozidae]), requires development of cultivars resistant or tolerant to infection or capable of reducing spread or both. We examined the influence that five experimental breeding clones of potato had on potato psyllids and their ability to vector Lso. The ability of these potato clones to resist aphids (green peach aphids, Myzus persicae Sulzer [Hemiptera: Aphididae]) also was examined. Due to the importance of host chemistry on plant-insect interactions, levels of primary metabolites of amino acids and sugars, as well as secondary metabolites including polyphenolics, terpenoids, and alkaloids were compared between breeding clones and a commercial cultivar. Findings for compound levels then were associated with observed changes in host susceptibility to psyllids or aphids. Psyllids oviposited less on three breeding clones than Atlantic, but no significant effects of breeding clones on psyllid feeding or choice were observed. Aphid reproduction was reduced on two clones relative to Atlantic. A05379-211 had greater sugar levels and postpsyllid amino acid levels than Atlantic. Total alkaloid and phenolic levels were greater in all breeding clones than Atlantic. Total terpenoid levels were greater in PALB03016-3 and PALB03016-6 than Atlantic, which might explain, in part, the observed resistance to psyllid oviposition and aphid reproduction. Overall, these results suggest that increased levels of certain metabolites in breeding clones could affect psyllid and aphid reproduction.

Phenology of the Potato Psyllid, Bactericera cockerelli (Hemiptera: Triozidae), and "Candidatus Liberibacter solanacearum" in Commercial Potato Fields in Idaho

Zebra chip disease (ZC) is an emerging disease of potato in which tubers are produced with striped necrotic patterns that make them unmarketable. ZC is associated with the bacterium "Candidatus Liberibacter solanacearum" (Lso), which is transmitted by the potato psyllid, Bactericera cockerelli (Šulc; Hemiptera: Triozidae). First found in Idaho during 2011, ZC now contributes to increased production costs each season via additional insecticide sprays. To clarify the extent and severity of the threat of ZC in Idaho, we sampled potato psyllids in commercial potato fields across the state over four growing seasons (2012-2015). All life stages of psyllids were sampled using a combination of methods (yellow sticky traps, vacuum samples, and leaf samples), and adult psyllids were tested for the presence of Lso by Polymerase Chain Reaction (PCR). Abundance of potato psyllids initially increased gradually over each growing season, then exhibited a sharp late-season rise and a sharp decline as most fields were being harvested. Abundance of psyllids was higher at warmer, lower elevation sites, but infestation onset did not differ between growing regions. Fewer psyllids were collected in vacuum samples than in sticky trap samples. Nymphs and eggs were found only late season and during years with high abundance of adults. Overall incidence of Lso was similar among all years but one. The results presented here clarify our understanding of the seasonal phenology of potato psyllids and Lso in Idaho potato fields and will aid in developing integrated management strategies against this important pest of potato.

Relative Abundance of Potato Psyllid Haplotypes in Southern Idaho Potato Fields During 2012 to 2015, and Incidence of 'Candidatus Liberibacter solanacearum' Causing Zebra Chip 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.

Population Dynamics of Released Potato Psyllids and their Bacteriliferous Status in Relation to Zebra Chip Incidence in Caged Field Plots

Potato psyllids vector 'Candidatus Liberibacter solanacearum' (Lso), the putative causal agent of potato zebra chip (ZC). Currently, sticky traps are the primary psyllid monitoring tools used by growers for making management decisions. However, the reliability of sticky traps in predicting psyllid numbers in potato fields has always been questioned. In 2013 and 2014, experiments were conducted in covered field plots at the Texas A&M AgriLife Research Station at Bushland to investigate the relationships among initial psyllid numbers, psyllids captured on sticky traps and their Lso status, and zebra chip incidence. Three densities of Lso-positive psyllids (5, 15, or 30/cage) were released under 2-week-old potato canopies with four replications in plot sizes of 7.6 to 9 m by 5 rows. Psyllids were released under the first plant in the center row and monitored weekly with a yellow sticky trap from the opposite end. Number of plants with zebra chip symptoms also was counted weekly beginning one month after infestation with psyllids. The total number of psyllids captured on sticky traps and disease incidence levels generally corresponded to the levels of psyllid density treatments (5 < 15 < 30), but the differences became more apparent toward the end of the experiments. Psyllid numbers in the different density treatments fluctuated more or less in synchrony over time, which appeared to reflect periodic emergence of new generations of psyllids. Initially, all captured psyllids tested positive for Lso. However, the proportions of psyllids testing positive declined dramatically after a few weeks, which suggested that the new generations of psyllids were devoid of Lso. Over all, less than 50% of captured psyllids tested positive for the pathogen. The decline in proportions of psyllids testing positive for Lso following successive generations has significant relevance to field situations and may partly explain why there are generally low percentages of Lso-positive psyllids under field conditions.

'Candidatus Liberibacter solanacearum' Titers in and Infection Effects on Potato Tuber Chemistry of Promising Germplasm Exhibiting Tolerance to Zebra Chip Disease

Long-term sustainable management of zebra chip (ZC) disease of potato requires development of tolerant or resistant germplasm. To this end, 283 potato varieties and breeding clones were infected with the ZC putative causal agent 'Candidatus Liberibacter solanacearum' (Lso) by potato psyllid vector inoculations in 2010, 2011, 2012, and 2013. Potato germplasm was then examined for development of fresh and fried ZC symptoms. Over multiple years 29 breeding clones exhibited little to no symptoms in freshly cut tuber slices, and five exhibited little to no symptoms in fried slices. These five presumed tolerant breeding clones were chosen for further screening to determine whether the lack of physiological responses to Lso infection was the cause of observed tolerance. To this end, tuber amino acid, sugar, and phenolic levels were compared between noninfected and Lso-infected plants. The five putative tolerant clones had less dramatic shifts in host physiology following Lso infection than the susceptible Atlantic cultivar. This suggested lack of host responses to Lso infection that result in major changes in tuber biochemistry is a potential mechanism of ZC resistance. However, the susceptible Atlantic cultivar did have consistently greater Lso titers compared with two of the tolerant entries, so for these reductions in Lso pathogen progression also might be a factor. Regardless, lack of host responses could still remain one trait that could be used to aid in selection of ZC-resistant potato varieties, as other tolerant lines had infection levels consistent with susceptible Atlantic cultivar. These results also suggest that germplasm derived from relatives of cultivated potato plants are viable sources of ZC disease resistance.

Use of Electrical Penetration Graph Technology to Examine Transmission of 'Candidatus Liberibacter solanacearum' to Potato by Three Haplotypes of Potato Psyllid (Bactericera cockerelli; Hemiptera: Triozidae)

The potato psyllid, Bactericera cockerelli (Šulc) (Hemiptera: Triozidae), is a vector of the phloem-limited bacterium ‘Candidatus Liberibacter solanacearum’ (Lso), the putative causal agent of zebra chip disease of potato. Little is known about how potato psyllid transmits Lso to potato. We used electrical penetration graph (EPG) technology to compare stylet probing behaviors and efficiency of Lso transmission of three haplotypes of potato psyllid (Central, Western, Northwestern). All haplotypes exhibited the full suite of stylet behaviors identified in previous studies with this psyllid, including intercellular penetration and secretion of the stylet pathway, xylem ingestion, and phloem activities, the latter comprising salivation and ingestion. The three haplotypes exhibited similar frequency and duration of probing behaviors, with the exception of salivation into phloem, which was of higher duration by psyllids of the Western haplotype. We manipulated how long psyllids were allowed access to potato (“inoculation access period”, or IAP) to examine the relationship between phloem activities and Lso transmission. Between 25 and 30% of psyllids reached and salivated into phloem at an IAP of 1 hr, increasing to almost 80% of psyllids as IAP was increased to 24 h. Probability of Lso-transmission was lower across all IAP levels than probability of phloem salivation, indicating that a percentage of infected psyllids which salivated into the phloem failed to transmit Lso. Logistic regression showed that probability of transmission increased as a function of time spent salivating into the phloem; transmission occurred as quickly as 5 min following onset of salivation. A small percentage of infected psyllids showed extremely long salivation events but nonetheless failed to transmit Lso, for unknown reasons. Information from these studies increases our understanding of Lso transmission by potato psyllid, and demonstrates the value of EPG technology in exploring questions of vector efficiency.

Silverleaf Nightshade (Solanum elaeagnifolium), a Reservoir Host for 'Candidatus Liberibacter solanacearum', the Putative Causal Agent of Zebra Chip Disease of Potato

Zebra chip disease of potato is caused by the bacterial pathogen 'Candidatus Liberibacter solanacearum' and is a growing concern for commercial potato production in several countries in North and Central America and New Zealand. 'Ca. L. solanacearum' is vectored by the potato psyllid Bactericera cockerelli, which transmits the pathogen to several cultivated and wild solanaceaous host plants. Silverleaf nightshade (SLN), Solanum elaeagnifolium, is a common weed in the Lower Rio Grande Valley of Texas and a host for both the potato psyllid and 'Ca. L. solanacearum'. SLN plants were successfully inoculated with 'Ca. L. solanacearum' under laboratory conditions. Retention studies demonstrated that 'Ca. L. solanacearum'-infected SLN planted in the field in January 2013, concurrent with commercial potato planting, retained the pathogen under field conditions throughout the year despite extensive dieback during summer. The presence of 'Ca. L. solanacearum' was confirmed in leaves, roots, and stolons of SLN plants collected the following year using polymerase chain reaction. Acquisition assays using B. cockerelli adults also revealed that SLN retained the pathogen. Transmission studies determined that B. cockerelli can acquire 'Ca. L. solanacearum' within a 2-week acquisition access period on 'Ca. L. solanacearum'-infected SLN and subsequently transmit the pathogen to potato. These results demonstrate that SLN plants can serve as a reservoir for 'Ca. L. solanacearum', providing a source of inoculum for B. cockerelli adults colonizing potato the next season. The presence of SLN plants all year round in the LRGV makes the weed an epidemiologically important host. These findings underscore the importance of eradicating or managing SLN plants growing in the vicinity of potato fields to prevent spread of 'Ca. L. solanacearum' and damage caused by zebra chip.

Significance and Epidemiological Aspects of Late-Season Infections in the Management of Potato Zebra Chip

Zebra chip (ZC) of potato is putatively caused by the fastidious, phloem-limited bacterium 'Candidatus Liberibacter solanacearum' (Lso), which is transmitted by the potato psyllid (Bactericera cockerelli). The disease, which significantly impacts both crop yield and quality, was first identified in the United States from south Texas in 2000. It reached epidemic levels in north Texas and certain production areas in Colorado, Nebraska, and New Mexico from 2004 to 2007 and it caused severe losses in fields in Oregon, Washington, and Idaho in 2011. The potato plant is susceptible to infection at all developmental stages, but disease management programs have focused on vector control through early and repeated insecticide applications, in an effort to minimize early to midseason infections which are most damaging. Growers often terminate spray programs 2 to 3 weeks prior to crop harvest due to lack of visible treatment effects on crop yield or quality. However, recent studies on vector transmission and host-pathogen interactions have revealed that late-season infections pose a significant, previously unrecognized, threat to crop quality. The pathogen can move from an infected leaf to tubers within 2 days; however, tubers infected less than 1 week before harvest will remain asymptomatic and the pathogen will be undetectable. When these tubers are placed into storage they are assumed to be disease free. However, Lso can continue to multiply in respiring tubers during storage, resulting in reduced tuber quality. Likewise, if plants become infected a few days before vines are killed, ZC can continue to develop in infected tubers before they are harvested. Perspectives on the significance of late-season infections and some of the more important issues associated with those infections are discussed.

Emergence of 'Candidatus Liberibacter solanacearum'-Infected Seed Potato in Relation to the Time of Infection

Zebra chip (ZC) is a disease of potato, putatively caused by the vectorborne bacterium 'Candidatus Liberibacter solanacearum'. Although ZC has been a major concern due its significant negative impact on both potato yield and quality, its effect on seed potato sprouting has been the subject of recent evaluations. The present study was conducted to determine whether variation in emergence is affected by the infection duration of 'Ca. L. solanacearum'-infected seed potato prior to harvest. Furthermore, changes in pathogen detectability and titer levels in late-season-infected plants also were evaluated during and after cold storage. The rate of ZC-affected seed potato emergence following cold storage was not affected by the time of infection in the field, and the majority of ZC-infected tubers failed to sprout. Time to "seedling" emergence also was significantly longer in seed potato from plants infected ≥2 weeks before harvest. The small percentage of plants that emerged from ZC-affected seed potato produced stunted, nonvigorous plants that often died after a few weeks. The rate of successful 'Ca. L. solanacearum' detection increased during cold storage, suggesting a continued 'Ca. L. solanacearum'-tuber interaction postharvest. After tubers were removed from cold storage and held at room temperature, 'Ca. L. solanacearum' titer started to increase. Although none of the tubers from plants infected 1 week before harvest exhibited any disease symptoms or tested positive for 'Ca. L. solanacearum' at harvest, up to 38% of these tubers tested positive following placement at room temperature after cold storage. Results of this study suggest that the role of seedborne ZC in disease epidemiology is likely to be insignificant. Furthermore, the findings of this study emphasized the importance of continued control measures until at least a week before harvest, and highlighted the need for improved methods of 'Ca. L. solanacearum' detection at harvest, especially in tubers infected late in the season.

Effects of Potato-Psyllid-Vectored 'Candidatus Liberibacter solanacearum' Infection on Potato Leaf and Stem Physiology

The bacterium 'Candidatus Liberibacter solanacearum' is associated with zebra chip disease (ZC), a threat to potato production in North America and New Zealand. It is vectored by potato psyllids. Previous studies observed that 'Ca. L. solanacearum' infection causes potato tubers to undergo ZC-symptom-associated shifts in physiology, such as increased levels of amino acids, sugars, and phenolics. However, little is known about how 'Ca. L. solanacearum' infections caused by psyllid vector feeding may affect metabolism in potato foliage and stems. This study compared metabolism in potato plants fed upon by 'Ca. L. solanacearum'-positive psyllids with potato plants not exposed to psyllids. Foliar levels of asparagine, aspartic acid, glutamine, fructose, glucose, sucrose, a ferulic acid derivative, and quinic acid were lower in 'Ca. L. solanacearum'-inoculated than noninfected plants. However, foliar levels of proline, serine, four phenolic compounds, and most terpenoids were greater in 'Ca. L. solanacearum'-inoculated than noninfected plants. Upper stem levels of asparagine and aspartic acid, upper and lower stem levels of ellagitannins and most monoterpenoids, and lower stem level of sesquiterpenoids were greater in 'Ca. L. solanacearum'-inoculated than noninfected plants. These results suggest that many defense-related terpenoid compounds might increase in plants which had psyllids inoculate 'Ca. L. solanacearum'. This could impact progression and spread of ZC.

Similarities and differences in physiological responses to 'Candidatus Liberibacter solanacearum' infection among different potato cultivars

Zebra chip disease (ZC), putatively caused by the fastidious bacterium 'Candidatus Liberibacter solanacearum', is a threat to potato growers worldwide. However, little is known about biochemical shifts in different potato genotypes in response to 'Ca. L. solanacearum' infection. To address this, 'Red La Soda', 'Russet Norkotah', and 'FL 1867' potato were infected with 'Ca. L. solanacearum' 4, 3, 2, and 1 weeks before harvest to observe variability in cultivar responses to 'Ca. L. solanacearum' infection. ZC symptoms, 'Ca. L. solanacearum' titers, and tuber biochemistry were assessed. Red La Soda tubers exhibited greater symptoms when infected for 4 weeks than Russet Norkotah or FL 1867 tubers. 'Ca. L. solanacearum' titers did not vary among cultivars. Tuber levels of amino acids, carbohydrates, and phenolics varied among cultivars but no consistent trends were observed. Individual amino acids and phenolics were greater in FL 1867 than Red La Soda, whereas others were greater in Red La Soda or Russet Norkotah than FL 1867. Most amino acids, carbohydrates, and phenolics were positively associated with infection duration and symptoms regardless of cultivar. Associations between most of the evaluated compounds and 'Ca. L. solanacearum' titer were positive in Red La Soda. However, no associations between 'Ca. L. solanacearum' quantity and compounds were observed in FL 1867 and Russet Norkotah.

Characterization of management and environmental factors associated with regional variations in potato zebra chip occurrence

Potato zebra chip (ZC), caused by the bacterial pathogen 'Candidatus Liberibacter solanacearum', which is vectored by the potato psyllid (Bactericera cockerelli), has caused widespread damage to U.S. potato production ever since its first discovery in south Texas in 2000. To determine the influence of environmental factors and management practices on ZC occurrence, data on management and meteorological variables, field locations, and psyllid counts were collected over a 3-year period (2010 to 2012) from six locations across the central United States (south Texas to Nebraska). At these locations, ZC-symptomatic plants were counted in 26 fields from systematically established 20 m × 30 m plots around the field edges and field interiors. Mean numbers of symptomatic plants per plot were classified into two intensity classes (ZC ≤ 3 or ZC > 3) and subjected to discriminant function and logistic regression analyses to determine which factors best distinguish between the two ZC intensity classes. Of all the variables, location, planting date, and maximum temperature were found to be the most important in distinguishing between ZC intensity classes. These variables correctly classified 88.5% of the fields into either of the two ZC-intensity classes. Logistic regression analysis of the individual variables showed that location accounted for 90% of the variations, followed by planting date (86%) and maximum temperature (70%). There was a low but significant (r = -0.44983, P = 0.0211) negative correlation between counts of psyllids testing positive for pathogen and latitudinal locations, indicating a south-to-north declining trend in counts of psyllids testing positive for the pathogen. A similar declining trend also was observed in ZC occurrence (r = -0.499, P = 0.0094).

Aspects of pathogen genomics, diversity, epidemiology, vector dynamics, and disease management for a newly emerged disease of potato: zebra chip

An overview is provided for the aspects of history, biology, genomics, genetics, and epidemiology of zebra chip (ZC), a destructive disease of potato (Solanum tuberosum) that represents a major threat to the potato industries in the United States as well as other potato-production regions in the world. The disease is associated with a gram-negative, phloem-limited, insect-vectored, unculturable prokaryote, 'Candidatus Liberibacter solanacearum', that belongs to the Rhizobiaceae family of α-Proteobacteria. The closest cultivated relatives of 'Ca. L. solanacearum' are members of the group of bacteria known as the α-2 subgroup. In spite of the fact that Koch's postulates sensu stricto have not been fulfilled, a great deal of progress has been made in understanding the ZC disease complex since discovery of the disease. Nevertheless, more research is needed to better understand vector biology, disease mechanisms, host response, and epidemiology in the context of vector-pathogen-plant interactions. Current ZC management strategies focus primarily on psyllid control. The ultimate control of ZC likely relies on host resistance. Unfortunately, all commercial potato cultivars are susceptible to ZC. Elucidation of the 'Ca. L. solanacearum' genome sequence has provided insights into the genetic basis of virulence and physiological and metabolic capability of this organism. Finally, the most effective, sustainable management of ZC is likely to be based on integrated strategies, including removal or reduction of vectors or inocula, improvement of host resistance to the presumptive pathogen and psyllid vectors, and novel gene-based therapeutic treatment.

Development of a loop-mediated isothermal amplification procedure as a sensitive and rapid method for detection of 'candidatus Liberibacter solanacearum' in potatoes and Psyllids

This study reports the development of a loop-mediated isothermal amplification procedure (LAMP) for polymerase chain reaction (PCR)-based detection of 'Candidatus Liberibacter solanacearum', the bacterial causal agent of potato zebra chip (ZC) disease. The 16S rDNA gene of 'Ca. Liberibacter solanacearum' was used to design a set of six primers for LAMP PCR detection of the bacterial pathogen in potato plants and the psyllid vector. The advantage of the LAMP method is that it does not require a thermocycler for amplification or agarose gel electrophoresis for resolution. Positive LAMP results can be visualized directly as a precipitate. The LAMP strategy reported here reliably detected 'Ca. Liberibacter solanacearum' and the closely related species 'Ca. Liberibacter asiaticus', the causative agent of huanglongbing disease of citrus, in plant DNA extracts. Although not as sensitive as quantitative real-time PCR, LAMP detection was equivalent to conventional PCR in tests of ZC-infected potato plants from the field. Thus, the LAMP method shows strong promise as a reliable, rapid, and cost-effective method of detecting 'Ca. Liberibacter' pathogens in psyllids and field-grown potato plants and tubers.