Effects of liberibacter-infective Bactericera cockerelli (Hemiptera: Triozidae) density on zebra chip potato disease incidence, potato yield, and tuber processing quality

In plant pathosystems involving insect vectors, disease spread, incidence, and severity often depend on the density of the vector population and its rate of infectivity with the disease pathogen. The potato psyllid, Bactericera cockerelli (Sulc), has recently been associated with zebra chip (ZC), an emerging and economically important disease of potato in the United States, Mexico, Central America, and New Zealand. "Candidatus Liberibacter solanacearum," a previously undescribed species of liberibacter has been linked to the disease and is transmitted to potato by B. cockerelli. Experiments were conducted under laboratory and field conditions to determine the impact of B. cockerelli density on ZC incidence, potato yield, and tuber processing quality. Insect densities ranging from one to 25 liberibacter-infective psyllids per plant were used during the experiments. Results showed that a single adult potato psyllid was capable of inoculating liberibacter to potato and causing ZC disease after a 72-h inoculation access period and was as damaging as 25 psyllids per plant. In addition, ZC-diseased plants showed a sharp reduction in tuber yield but the disease response was independent of the density of psyllids. Furthermore, both glucose and sucrose were found to have highly elevated concentrations in ZC-diseased potato tubers compared with noninfected ones and psyllid density did not vary the response. The high reducing sugar concentrations found in ZC-infected potato tubers are believed to be responsible for browning and reduced quality in processed ZC-infected tubers. This information could help ZC-affected potato producers in making effective management decisions for this serious disease.     

Oral delivery of double-stranded RNAs and siRNAs induces RNAi effects in the potato/tomato psyllid, Bactericerca cockerelli

The potato/tomato psyllid, Bactericerca cockerelli (B. cockerelli), and the Asian citrus psyllid, Diaphorina citri (D. citri), are very important plant pests, but they are also vectors of phloem-limited bacteria that are associated with two devastating plant diseases. B. cockerelli is the vector of Candidatus Liberibacter psyllaurous (solanacearum), which is associated with zebra chip disease of potatoes, and D. citri is the vector of Ca. Liberibacter asiaticus, which is associated with the Huanglongbing (citrus greening) disease that currently threatens the entire Florida citrus industry. Here we used EST sequence information from D. citri to identify potential targets for RNA interference in B. cockerelli. We targeted ubiquitously expressed and gut-abundant mRNAs via injection and oral acquisition of double-stranded RNAs and siRNAs and were able to induce mortality in recipient psyllids. We also showed knockdown of target mRNAs, and that oral acquisition resulted primarily in mRNA knockdown in the psyllid gut. Concurrent with gene knockdown was the accumulation of target specific ～ 21 nucleotide siRNAs for an abundant mRNA for BC-Actin. These results showed that RNAi can be a powerful tool for gene function studies in psyllids, and give support for continued efforts for investigating RNAi approaches as possible tools for psyllid and plant disease control.     

Vector transmission efficiency of liberibacter by Bactericera cockerelli (Hemiptera: Triozidae) in zebra chip potato disease: effects of psyllid life stage and inoculation access period

Successful transmission of plant pathogens by insects depends on the vector inoculation efficiency and how rapidly the insect can effectively transmit the pathogen to the host plant. The potato psyllid, Bactericera cockerelli (Sulc), has recently been found to transmit "Candidatus Liberibacter solanacearum," a bacterium associated with zebra chip (ZC), an emerging and economically important disease of potato in several parts of the world. Currently, little is known about the epidemiology of ZC and its vector's inoculation capabilities. Studies were conducted in the field and laboratory to 1) assess transmission efficiency of potato psyllid nymphs and adults; 2) determine whether psyllid inoculation access period affects ZC incidence, severity, and potato yield; and 3) determine how fast the psyllid can transmit liberibacter to potato, leading to ZC development. Results showed that adult potato psyllids were highly efficient vectors of liberibacter that causes ZC and that nymphs were less efficient than adults at transmitting this bacterium. It was also determined that inoculation access period had little influence on overall ZC disease incidence, severity, and resulting yield loss. Moreover, results showed that exposure of a plant to 20 adult potato psyllids for a period as short as 1 h resulted in ZC symptom development. Furthermore, it was shown that a single adult potato psyllid was capable of inoculating liberibacter to potato within a period as short as 6 h, thereby inducing development of ZC. This information will help in developing effective management strategies for this serious potato disease.     

Identification and location of symbionts associated with potato psyllid (Bactericera cockerelli) lifestages

The potato psyllid (Bactericera cockerelli, Sulc) is an invasive pest of solenaceous plants including potatoes (Solanum tuberosum L.)and tomatoes (Solanum lycopersicum L.). The insect transmits the phytopathogen Candidatus Liberibacter solanacearum, which has been identified as the causal agent of Zebra Chip in potatoes. The microbiome of the potato psyllid provides knowledge of the insect's bacterial makeup which enables researchers to develop targeted biological control strategies. In this study, the microbes associated with four B. cockerelli life stages were evaluated by 16S bTEFAP pyrosequencing. The sequences were compared with a 16S-rDNA database derived from NCBI's GenBank. Some bacteria identified are initial discoveries. Species of Wolbachia, Rhizobium, Gordonia, Mycobacterium, Xanthomonas and others were also detected and an assessment of the microbiome associated with B. cockerelli was established.     

Life history and life tables of Bactericera cockerelli (Hemiptera: Psyllidae) on potato under laboratory and field conditions in the lower Rio Grande Valley of Texas

Effective management of potato 'Zebra Chip' (ZC) disease caused by Cadidatus Liberibacter psyllaurous (syn. solanacearum) depends on the management of its insect vector insect, potato psyllid, Bactericera cockerelli (Sulc) (Hemiptera: Psyllidae). To elucidate the age-specific population dynamics of B. cockerelli, the life-table parameters were determined on potato, Solanum tuberosum L., under both laboratory and field conditions in the Lower Rio Grande Valley (LRGV) of Texas. Generally, survival, fecundity, and longevity of B. cockerelli were significantly greater under laboratory than under field conditions. The mortality under laboratory conditions was mainly due to natural intrinsic mortality. However, under field conditions, most (83.2%) B. cockerelli were missing, and of those that were not, they developed slower, and had shorter preoviposition period, shorter oviposition period, shorter longevity, lower fecundity, and higher mortality than those under laboratory conditions. As a result, most of the life-table parameters of B. cockerelli, including the intrinsic rate of increase, finite rate of increase, and net reproductive rate, were significantly lower in the field under the environmental conditions of the LRGV of Texas than in the laboratory. The information could help increase our understanding of the epidemiology of the ZC diseases associated with the pathogens transmitted by this insect pest.     

Zebra chip disease incidence on potato is influenced by timing of potato psyllid infestation, but not by the host plants on which they were reared

Abstract  The Zebra chip (ZC) syndrome is an emerging disease of potato and a major threat to the potato industry. The potato psyllid, Bactericerca cockerelli (Sulc) is believed to be a vector of the ZC pathogen, which is now thought to be Candidatus Liberibacter, a bacterium. To further understand the relationship between potato psyllid infestation and ZC disease expression, healthy potato plants at different growth stages (4, 6 and 10 weeks after germination) were exposed separately to potato psyllids that were separately reared on four solanaceous hosts plants (potato, tomato, eggplant or bell pepper) for more than 1 year. ZC symptoms, leaf rates and total nonstructural carbohydrate accumulation in leaves and tubers of healthy and psyllid-infested plants were monitored and recorded. Typical ZC symptoms were observed in leaves and tubers of all plants exposed to potato psyllids regardless of the host plant on which they were reared. This was also accompanied by significant reductions in net photosynthetic rate. Caged potato plants without exposure to potato psyllids (uninfested controls) did not show any ZC symptom in both foliage and in harvested tubers. Foliage damage and ZC expression were most severe in the potato plants that were exposed to potato psyllids 4 weeks after germination compared to plants infested at later growth stages. Tubers from potato psyllid-infested plants had significantly higher levels of reducing sugars (glucose) and lower levels of starch than those in healthy plants, indicating that potato psyllid infestation interfered with carbohydrate metabolism in either leaves or tubers, resulting in ZC expression.     

Seasonal population dynamics of the potato psyllid (Hemiptera: Triozidae) and its associated pathogen "Candidatus Liberibacter solanacearum" in potatoes in the southern great plains of North America

The potato psyllid, Bactericera cockerelli (Sulc) (Hemiptera: Triozidae), and its associated pathogen "Candidatus Liberibacter solanacearum" (Ca. L. solanacearum), the putative causal agent of zebra chip (ZC) disease in potatoes (Solanum tuberosum L.), were sampled in commercial potato fields and untreated control plots for 3 yr in multiple locations in Texas, Kansas, Nebraska, and Colorado. Populations of the potato psyllid varied across years and across potato growing regions. However, the percentage of potato psyllids infected with Ca. L. solanacearum although variable across years, was consistently highest in the Lower Rio Grande Valley of Texas (LRGV), the reported overwintering location for this pest. The numbers of Ca. L. solanacearum-infected psyllids collected on field traps and large nymphs counted on leaf samples were both positively correlated with the final percentage of ZC in tubers. In the LRGV, where vector and disease pressure is the highest, population levels of immature life stages of the psyllid and percentage of ZC differed greatly between commercial and untreated fields. These results show that the pest management program that was used can be effective at controlling development of the psyllid and ultimately reducing the incidence of ZC.     

Impact of potato psyllid (Hemiptera: Triozidae) feeding on free amino acid composition in potato

Abstract The impacts of potato psyllid (Bactericera cockerelli) feeding on potato foliage on the free amino acids (FAAs) composition in potato leaf and tubers were determined under greenhouse conditions. The free amino acids in plant extracts were separated by high‐performance liquid chromatography, and in both leaf and tuber samples, at least 17 FAAs were detected. Psyllid feeding significantly changed the levels of several FAAs in both leaf and tuber samples. The concentration of leucine increased 1.5‐fold, whereas that of serine and proline increased 2‐ and 3‐fold, respectively. In contrast, the concentrations of glutamic acid, aspartic acid and lyscine were significantly reduced by 42.0%, 52.1% and 27.5%, respectively. There were also significant changes in the levels of FAAs in the Zebra chip (ZC) infected tubers compared with the healthy tubers, and the levels of six of the FAAs increased, and the levels of nine of the FAAs decreased. The results from this study indicate that potato psyllid causes major changes in free amino acid composition of plant tissues, and this change in plant metabolism may contribute to the plant stress as indicated by increased levels of proline in the leaves and hence promoting the development of plant diseases such as ZC disease.     

A new Huanglongbing Species, "Candidatus Liberibacter psyllaurous," found to infect tomato and potato, is vectored by the psyllid Bactericera cockerelli (Sulc)

A new huanglongbing (HLB) "Candidatus Liberibacter" species is genetically characterized, and the bacterium is designated "Candidatus Liberibacter psyllaurous." This bacterium infects the psyllid Bactericera cockerelli and its solanaceous host plants potato and tomato, potentially resulting in "psyllid yellowing." Host plant-dependent HLB transmission and variation in psyllid infection frequencies are found.     

Molecular and physiological properties associated with zebra complex disease in potatoes and its relation with Candidatus Liberibacter contents in psyllid vectors

Zebra complex (ZC) disease on potatoes is associated with Candidatus Liberibacter solanacearum (CLs), an α-proteobacterium that resides in the plant phloem and is transmitted by the potato psyllid Bactericera cockerelli (?ulc). The name ZC originates from the brown striping in fried chips of infected tubers, but the whole plants also exhibit a variety of morphological features and symptoms for which the physiological or molecular basis are not understood. We determined that compared to healthy plants, stems of ZC-plants accumulate starch and more than three-fold total protein, including gene expression regulatory factors (e.g. cyclophilin) and tuber storage proteins (e.g., patatins), indicating that ZC-affected stems are reprogrammed to exhibit tuber-like physiological properties. Furthermore, the total phenolic content in ZC potato stems was elevated two-fold, and amounts of polyphenol oxidase enzyme were also high, both serving to explain the ZC-hallmark rapid brown discoloration of air-exposed damaged tissue. Newly developed quantitative and/or conventional PCR demonstrated that the percentage of psyllids in laboratory colonies containing detectable levels of CLs and its titer could fluctuate over time with effects on colony prolificacy, but presumed reproduction-associated primary endosymbiont levels remained stable. Potato plants exposed in the laboratory to psyllid populations with relatively low-CLs content survived while exposure of plants to high-CLs psyllids rapidly culminated in a lethal collapse. In conclusion, we identified plant physiological biomarkers associated with the presence of ZC and/or CLs in the vegetative potato plant tissue and determined that the titer of CLs in the psyllid population directly affects the rate of disease development in plants.     

The complete genome sequence of 'Candidatus Liberibacter solanacearum', the bacterium associated with potato zebra chip disease

Zebra Chip (ZC) is an emerging plant disease that causes aboveground decline of potato shoots and generally results in unusable tubers. This disease has led to multi-million dollar losses for growers in the central and western United States over the past decade and impacts the livelihood of potato farmers in Mexico and New Zealand. ZC is associated with 'Candidatus Liberibacter solanacearum', a fastidious alpha-proteobacterium that is transmitted by a phloem-feeding psyllid vector, Bactericera cockerelli Sulc. Research on this disease has been hampered by a lack of robust culture methods and paucity of genome sequence information for 'Ca. L. solanacearum'. Here we present the sequence of the 1.26 Mbp metagenome of 'Ca. L. solanacearum', based on DNA isolated from potato psyllids. The coding inventory of the 'Ca. L. solanacearum' genome was analyzed and compared to related Rhizobiaceae to better understand 'Ca. L. solanacearum' physiology and identify potential targets to develop improved treatment strategies. This analysis revealed a number of unique transporters and pathways, all potentially contributing to ZC pathogenesis. Some of these factors may have been acquired through horizontal gene transfer. Taxonomically, 'Ca. L. solanacearum' is related to 'Ca. L. asiaticus', a suspected causative agent of citrus huanglongbing, yet many genome rearrangements and several gene gains/losses are evident when comparing these two Liberibacter. species. Relative to 'Ca. L. asiaticus', 'Ca. L. solanacearum' probably has reduced capacity for nucleic acid modification, increased amino acid and vitamin biosynthesis functionalities, and gained a high-affinity iron transport system characteristic of several pathogenic microbes.     

Sampling and interpretation of psyllid nymph counts in potatoes

Development of effective management practices for insect pests relies heavily on sampling methods to accurately detect and quantify emerging populations. Herein we describe a novel method to extract and count nymphs of potato psyllid, Bactericera cockerelli (?ulc) (Hemiptera: Triozidae), from leaves in commercial fields of potato, Solanum tuberosum L. (Solanaceae). The proposed sampling method (referred to as the leaf washing method, LWM) consists of: (1) immersing samples of infested leaves in cold water to remove dust and sand; (2) immersing leaves in >85 °C water for 5 s; (3) extracting psyllid nymphs from heated water by passing it through a sampling unit composed of a funnel with a fine mesh‐organza fabric inside and attached to a vacuum pump; and (4) removing organza fabric and count psyllid nymphs under a stereoscope. With five sampling units operating simultaneously, one person is able to process and count immatures from about 400 leaf samples in two normal work days. As examples of applications, the LWM was used to characterize: (1) the vertical distribution of potato psyllid nymphs in the potato canopy; and (2) the spatio‐temporal distribution of potato psyllid nymphs in three potato varieties. Using LWM, we showed that psyllid nymphs were most predominant in the middle portion of the canopy and that spatio‐temporal distributions of nymphs varied among potato varieties.     

Association of Bactericera cockerelli (Homoptera: Psyllidae) with "zebra chip," a new potato disease in southwestern United States and Mexico

A new defect of potato, Solanum tuberosum L., "zebra chip," so named for the characteristic symptoms that develop in fried chips from infected potato tubers, has recently been documented in several southwestern states of the United States, in Mexico, and in Central America. This defect is causing millions of dollars in losses to both potato producers and processors. Zebra chip plant symptoms resemble those caused by potato purple top and psyllid yellows diseases. Experiments were conducted to elucidate the association between the psyllid Bactericera cockerelli (Sulc) (Homoptera: Psyllidae) and zebra chip by exposing clean potato plants to this insect under greenhouse and field conditions. Potato plants and tubers exhibiting zebra chip symptoms were tested for phytoplasmas by polymerase chain reaction. Potato psyllids collected from infected potato fields also were tested. Results indicated that there was an association between the potato psyllid and zebra chip. Plants exposed to psyllids in the greenhouse and field developed zebra chip. In the greenhouse, 25.8 and 59.2% of tubers exhibited zebra chip symptoms in the raw tubers and fried chips, respectively. In the field, 15 and 57% of tubers showed symptoms in raw tubers and chips, respectively. No zebra chip was observed in tubers from plants that had not been exposed to psyllids, either in the greenhouse or field. No phytoplasmas were detected from potato plants or tubers with zebra chip symptoms, suggesting that these pathogens are not involved in zebra chip. Of the 47 samples of potato psyllids tested, only two tested positive for the Columbia Basin potato purple top phytoplasma.     

Effects of insecticides on behavior of adult Bactericera cockerelli (Hemiptera: Triozidae) and transmission of Candidatus Liberibacter psyllaurous

The potato psyllid, Bactericera cockerelli (Sulc) (Hemiptera: Triozidae), is a serious pest of potatoes (Solanum tuberosum L.) that can cause yield loss by direct feeding on crop plants and by vectoring a bacterial pathogen, Candidatus Liberibacer psyllaurous. Current pest management practices rely on the use of insecticides to control the potato psyllid to lower disease incidences and increase yields. Although many studies have focused on the mortality that insecticides can cause on potato psyllid populations, little is known regarding the behavioral responses of the potato psyllid to insecticides or whether insecticides can decrease pathogen transmission. Thus, the objectives of this study were to determine the effects of insecticides on adult potato psyllid behaviors, the residual effects of insecticides on potato psyllid behaviors over time, and effects of these insecticides on Ca. L. psyllaurous transmission. Insecticides tested included imidacloprid, kaolin particle film, horticultural spray oil, abamectin, and pymetrozine. All insecticides significantly reduced probing durations and increased the amount of time adult psyllids spent off the leaflets, suggesting that these chemicals may be deterrents to feeding as well as repellents. Nonfeeding behaviors such as tasting, resting, and cleaning showed variable relationships with the different insecticide treatments over time. The insecticides imidacloprid and abamectin significantly lowered transmission of Ca. L. psyllaurous compared with untreated controls. The implications of our results for the selection of insecticides useful for an integrated pest management program for potato psyllid control are discussed.     

Differential responses to feeding by the tomato/potato psyllid between two tomato cultivars and their implications in establishment of injury levels and potential of damaged plant recovery

An invasive new biotype of the tomato/potato psyllid （Bactericera [Paratrioza] cockerelli [Sulc.]） （Homoptera： Psyllidae） recently has caused losses exceeding 50% on fresh market tomatoes in western North America. Despite these extensive losses, little is known regarding the threshold levels at which populations must be suppressed in order to prevent economic losses. A series of experiments were therefore designed using combinations of two common tomato cultivars （QualiT 21 and Yellow Pear）, five pest-densities （0, 20, 30, 40 and 50 nymphs/plant）, and three feeding-duration （5 days, 10 days, and lifetime） treatments to test the relative importance of pest density, feeding period, and cumulative psyllid-days to establish economic threshold levels for psyllids. The cultivars differed considerably in their response to the toxin injected by the psyllid nymphs. ‘Yellow Pear＇ plants could recover from feeding by up to 40 nymphs for as long as 10 d, whereas ‘QualiT 21＇ plants were irreparably damaged by densities of 20 nymphs feeding for only 5 days. On ‘Yellow Pear＇, all plant measurements such as the number of yellow leaves and plant height were significantly better correlated with cumulative psyUid-days than with either pest density or feeding duration. On ‘QualiT 21 ＇, all plant measurements other than the number of yellow leaflets and leaves were significantly better correlated with pest density than with feeding duration or cumulative psyUid-days, and pest density was a better predictor of psyUid damage. Potential reasons for the variable responses between cultivars and the implications for psyllid sampling and integrated pest management are discussed.     

Evaluation of potato tuber moth (Lepidoptera: Gelechiidae) resistance in tubers of Bt-cry5 transgenic potato lines

The potato tuber moth, Phthorimaea operculella (Zeller), in tropical and subtropical countries, is the most destructive pest of potato, Solanum tuberosum L. The larvae attack foliage and tubers in the field and in storage. The purpose of this study was to evaluate the efficacy of a Bt-cry5 transgene to control the potato tuber moth in tuber tissues. Tuber bioassays using stored (11-12 mo old) and newly harvested tubers of Bt-cry5-Lemhi Russet and Bt-cry5-Atlantic potato lines showed up to 100% mortality of 1st instars. Mortality was lowest in the newly harvested tubers of Bt-cry5-Atlantic lines (47.1-67.6%). Potato tuber moth mortality was 100% in the Bt-cry5-Spunta lines that were transformed with Bt-cry5 gene controlled by the CaMV 35S promoter (pBIML5 vector) and in 2 of 3 lines transformed with Bt-cry5 gene controlled by the Gelvin super promoter (pBIML1 vector). The transgenic Spunta lines expressing Bt-cry5 controlled by the patatin promoter (pBMIL2 vector) showed the lowest tuber moth mortality (25.6 and 31.1%). The Bt-cry5 transgenic lines with high tuber expression of B. thuringiensis have value in an integrated pest management system to control potato tuber moth.     

Growth and yield of potato plants inoculated with rhizosphere bacteria

Pot and field experiments on small plots were carried out to study the effect of isolates ofPseudomonas fluorescens-putida bacteria, applied to potato tubers, on the growth response of potato plants. Inoculation of tuber pieces with the isolates in pot experiments caused a better growth of young potato plant (111 % increase). The effect depended on the viable count in the bacterial suspension and on growth conditions of the plants. Inoculation of potato seed pieces before planting in field experiments caused a 4 –30 % improvement in plant growth and tuber yield. The complex action of inoculation with rhizobacteria apparently caused changes in the microflora colonizing the roots and stolons, which in turn brought about a better growth and yield.     

Evaluation of the parasitoid Copidosoma koehleri for biological control of the potato tuber moth,Phthorimaea operculella,in Israeli potato fields

The potato tuber moth Phthorimaea operculella (Zeller) (Lepidoptera: Gelechiidae) is a major agricultural pest of solaneceous crops in warm countries worldwide. The encyrtid polyembryonic parasitoid Copidosoma koehleri (Blanchard) has been successfully introduced for biological control of the moth in potato fields in South Africa and Australia; however, augmentative releases of the parasitoid in trial plots and in commercial potato fields in Israel did not reduce pest populations or infestation levels more than chemical treatment. Copidosoma koehleri accounted for 4–5% of parasitism on tuber moth caterpillars, while most parasitism was due to local species of larval parasitoids. The abundance and composition of local parasitoids did not differ between C. koehleri release plots and conventionally treated control plots. These findings can be interpreted as failure of the introduced parasitoids to survive and locate their hosts, or as mortality of C. koehleri within hosts in the field. Sentinel hosts, placed in trial plots and collected after 24 h, were rarely parasitised by C. koehleri, supporting the first interpretation. To test the second hypothesis, hosts parasitised by C. koehleri were placed in field plots for a week, collected, and reared out in the laboratory. The emergence rates of C. koehleri from these hosts resembled those of lab-reared controls, suggesting that mortality within hosts in the field is not a major cause of C. koehleri's poor biocontrol performance.