Venkatesan G. Sengoda

Association of “Candidatus Liberibacter solanacearum” with the Psyllid, Trioza apicalis (Hemiptera: Triozidae) in Europe

ABSTRACT The psyllid Trioza apicalis Förster (Hemiptera: Triozidae) is a serious pest of carrots, Daucus carota L., in Europe. Carrots exhibiting symptoms of psyllid damage were observed in commercial fields in southern Finland in 2008. Symptoms in affected plants included leaf curling, yellow and purple discoloration of leaves, stunted growth of shoots and roots, and proliferation of secondary roots, Mechanisms by which T. apicalis induces symptoms in plants are not understood, and no plant pathogens have yet been associated with this insect, Given recent association of liberibacter with several crops affected by psyllids, an investigation on whether this bacterium is associated with T. apicalis was conducted, Polymerase chain reaction (PCR) primer pairs OA2/OI2c and LsoF/OI2c, specific for 16S rRNA gene from “Candidatus Liberibacter solanacearum,” generated amplicons of 1,168 bp and 1,173 bp, respectively, from DNA extracted from field-collected psyllids (61 and 36.6%, respectively), laboratory-reared psyllids (70 and 33.3%, respectively), field-collected petioles from symptomatic carrots (80 and 55%, respectively), and laboratory-grown carrots (100% for both primer pairs). In contrast, no PCR products were detected in DNA extracted from insect-free plants. The DNA sequences of amplicons of the genes encoding liberibacter 16S rRNA from psyllids and carrots were identical. DNA of the 16S rRNA gene sequences determined from carrots and psyllids were 99.9% identical to analogous sequences of “Ca. L. solanacearum” amplified from several solanaceous crops and the psyllid Bactericera cockerelli (Sulc), a vector of this bacterium, This is the first report of a plant pathogen associated with T. apicalis and the second known psyllid species associated with “Ca. L. solanacearum.”

Vector Transmission Efficiency of Liberibacter by Bactericera cockerelli (Hemiptera: Triozidae) in Zebra Chip Potato Disease: Effects of Psyllid Life Stage and Inoculation Access Period

ABSTRACT 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 (Šulc), 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 vectors 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.

Phenotypic and Etiological Differences Between Psyllid Yellows and Zebra Chip Diseases of Potato

Both potato psyllid yellows and zebra chip (ZC) potato diseases are associated with the potato psyllid, Bactericera cockerelli (Sulc). Aboveground plant symptoms of both diseases are similar but there is a difference in symptoms in potato tubers. ZC has recently been associated with a new species of the bacterium liberibacter, ‘Candidatus Liberibacter solanacearum’, also known as ‘Ca. Liberibacter psyllaurous’. Mechanisms by which the potato psyllid might cause either ZC or potato psyllid yellows symptoms are not understood. Insect transmission studies were conducted to demonstrate psyllid vectoring of both diseases and to compare symptoms and development of the two diseases. Potato plants were exposed to both liberibacter-free and liberibacter-carrying potato psyllids and later evaluated for plant and tuber symptoms. These plants and tubers were then tested for liberibacter by polymerase chain reaction (PCR). In addition, potato plants exhibiting severe psyllid yellows/ZC-like symptoms were collected from a commercial potato field heavily infested with the potato psyllid and tested for liberibacter. PCR detected ‘Ca. Liberibacter solanacearum’ in ZC symptomatic plants and tubers resulting from exposure to liberibacter-carrying psyllids. Despite development of foliar symptoms that resemble those of ZC in plants exposed to liberibacter-free psyllids, no liberibacter was detected in these plants with psyllid yellows. Moreover, tubers from these plants with psyllid yellows did not exhibit any symptoms of ZC infection and tested negative for the bacterium. No liberibacter was detected in plants or tubers collected from the psyllid-infested potato field, suggesting that the observed symptoms were due to psyllid yellows. Furthermore, potato plants that were infected with liberibacter died sooner than plants that were infected with psyllid yellows. Although an association between liberibacter and ZC has been established, no pathogen is yet associated with potato psyllid yellows and mechanisms by which psyllid yellows symptoms are induced by the potato psyllid remain unclear.ResumenLas dos enfermedades de la papa, el amarillamiento de la papa por psílidos y zebra chip (ZC), están asociadas con el psílido de la papa Bactericera cockerelli (Sulc). Los síntomas aéreos de la planta por ambas enfermedades son similares, pero hay una diferencia en los síntomas del tubérculo. La ZC se ha asociado recientemente con una nueva especie de bacteria liberibacter, “Candidatus Liberibacter solanacearum” , también conocida como “Ca. Liberibacter psyllaurous”. No se han entendido los mecanismos por los cuales el psílido de la papa puede causar los síntomas, ya sea de la ZC o el amarillamiento de la papa por psílidos. Se condujeron estudios de la transmisión por insectos para demostrar la transmisión de ambas enfermedades por psílidos y para comparar los síntomas y el desarrollo de ambas enfermedades. Se expusieron plantas de papa a psílidos de la papa, tanto libres como con liberibacter, y se evaluaron posteriormente los síntomas de la planta y del tubérculo. Estas plantas y tubérculos se probaron después para liberibacter con la reacción en cadena de la polimerasa (PCR). Además, se colectaron plantas de papa que exhibieron síntomas severos de amarillamiento por psílidos/ZC, de un campo comercial de papa severamente infestado con el psílido de la papa, y se probaron para liberibacter. La PCR detectó “Ca. Liberibacter solanacearum” en plantas y tubérculos con síntomas de ZC, como resultado de su exposición a los psílidos con liberibacter. A pesar del desarrollo de síntomas foliares parecidos a los de ZC en plantas expuestas a los psílidos libres de liberibacter, no se detectó liberibacter en estas plantas con amarillamiento por psílidos. Aún mas, los tubérculos de estas plantas con amarillamiento por psílidos no exhibieron síntomas de infección por ZC y resultaron negativas a la bacteria. No se detectó liberibacter en plantas o tubérculos colectados del campo de papa infestado por psílidos, lo cual sugiere que los síntomas observados fueron debidos al amarillamiento por psílidos. Incluso, las plantas infectadas con liberibacter murieron más pronto que las infectadas con amarillamiento por psílidos. Aún cuando se ha establecido una asociación entre liberibacter y ZC, no se ha asociado a algún patógeno con el amarillamiento de la papa por psílidos y permanecen si aclararse los mecanismos por los cuales se inducen los síntomas del amarillamiento por el psílido de la papa.

A new haplotype of "Candidatus Liberibacter solanacearum" identified in the Mediterranean region

Abstract“Candidatus Liberibacter solanacearum”, a phloem-limited and Gram-negative bacterium that is spread from infected to healthy plants by psyllid insect vectors, is an economically important pathogen of solanaceous and carrot crops in the Americas, New Zealand and Europe. Three haplotypes of “Ca. L. solanacearum” have previously been described, two (LsoA and LsoB) in relation to solanaceous crops in the Americas and New Zealand and the third (LsoC) to carrots in Finland. Herein, we describe a fourth haplotype of this ‘Candidatus Liberibacter’ species (LsoD), also associated with carrots, but from Spain and the Canary Islands and vectored by the psyllid Bactericera trigonica. In addition, LsoC was confirmed in carrot and psyllid samples recently collected from Sweden and Norway. Phylogenetic analysis of the 16S rRNA gene suggests that two of the haplotypes, one in the Americas and the other in northern Europe are closer to each other in spite of a large geographic separation and host differences. Furthermore, during this study, potatoes with symptoms of zebra chip disease recently observed in potato crops in Idaho, Oregon and Washington states were analyzed for haplotype and were found to be positive for LsoA. This liberibacter haplotype was found in psyllids associated with the diseased potato crops as well. This finding contrasts with an earlier report of LsoB from psyllids in Washington which came from a laboratory colony originally collected in Texas.

Haplotypes of the Potato Psyllid, Bactericera cockerelli, on the Wild Host Plant, Solanum dulcamara, in the Pacific Northwestern United States

Abstract‘Candidatus Liberibacter solanacearum’ (Lso) is a bacterium that infects solanaceous crops and causes plant decline and yield losses, especially in potato and tomato. Lso is transmitted to these hosts by the potato psyllid (Bactericera cockerelli Sulc) vector. B. cockerelli host plants are not limited to crop plants, but also include many wild, solanaceous weeds. These wild hosts could potentially impact overwintering and breeding of the psyllids and serve as reservoirs for Lso. In the Pacific Northwestern United States, B. cockerelli was recently reported to overwinter on bittersweet nightshade (Solanum dulcamara L.). The present study utilized high resolution melting analysis of the B. cockerelli mitochondrial cytochrome c oxidase I gene to assess the psyllid populations occurring on S. dulcamara during the summer and winter months in Washington, Oregon, and Idaho. This technique has previously been used to analyze the cytochrome c oxidase I gene of B. cockerelli, and has identified four psyllid haplotypes. Lso infection was also determined for the psyllids collected from S. dulcamara. During both the summer and the winter months in the Pacific Northwest, the Northwestern psyllid haplotype was the predominant population found living on S. dulcamara. However, low levels of the Western psyllid population were also present in Washington and Oregon during the same period. No overwintering psyllids tested were Lso-infected, suggesting that these populations do not pose an imminent threat of Lso transmission to newly emerging potatoes and other solanaceous crops in the region, unless a source of Lso becomes available.Resumen‘Candidatus Liberibacter solanacearum’ (Lso) es una bacteria que infecta a cultivos de solanáceas y causa abatimiento y pérdida de cosechas, especialmente en papa y tomate. Lso se transmite a estos hospedantes por el vector psílido de la papa (Bactericera cockerelli Sulc). Las plantas hospederas de B. cockerelli no se limitan a especies cultivadas, sino que también incluyen muchas malezas silvestres solanáceas. Estas hospedantes silvestres pudieran impactar potencialmente la invernación y apareamiento de los psílidos y servir como reservorios para Lso. En el Noroeste del Pacífico de los Estados Unidos de América se ha reportado recientemente a B. cockerelli invernando en la planta “uva del diablo”, “dulcamara” o “matagallinas” (Solanum dulcamara L.). En el presente estudio se utilizó un análisis de fusión de alta resolución del gen mitocondrial del citocromo c oxidasa de B. cockerelli para analizar las poblaciones del psílido que se presentan en S. dulcamara durante los meses del verano e invierno en Washington, Oregon y Idaho. Se ha utilizado previamente esta técnica para analizar el gen mencionado, y ha identificado cuatro haplotipos del psílido. También se determinó la infección por Lso en psílidos colectados de S. dulcamara. Durante los meses de verano e invierno en el Pacífico del Noroeste, el haplotipo del psílido del Noroeste era la población dominante que se encontraba viviendo en S. dulcamara. No obstante, también se encontraba, aunque en bajos niveles de la población, el psílido del Oeste en Washington y Oregon durante el mismo período. Psílidos no invernantes probados estaban infectados de Lso, sugiriendo que estas poblaciones no representan una amenaza inminente de transmisión de Lso a papas de nueva emergencia y a otros cultivos de solanáceas en la región, a menos que una fuente de Lso estuviera disponible.

Latent Period and Transmission of “Candidatus Liberibacter solanacearum” by the Potato Psyllid Bactericera cockerelli (Hemiptera: Triozidae)

“Candidatus Liberibacter solanacearum” (Lso) is an economically important pathogen of solanaceous crops and the putative causal agent of zebra chip disease of potato (Solanum tuberosum L.). This pathogen is transmitted to solanaceous species by the potato psyllid, Bactericera cockerelli (Šulc), but many aspects of the acquisition and transmission processes have yet to be elucidated. The present study was conducted to assess the interacting effects of acquisition access period, incubation period, and host plant on Lso titer in psyllids, the movement of Lso from the alimentary canal to the salivary glands of the insect, and the ability of psyllids to transmit Lso to non-infected host plants. Following initial pathogen acquisition, the probability of Lso presence in the alimentary canal remained constant from 0 to 3 weeks, but the probability of Lso being present in the salivary glands increased with increasing incubation period. Lso copy numbers in psyllids peaked two weeks after the initial pathogen acquisition and psyllids were capable of transmitting Lso to non-infected host plants only after a two-week incubation period. Psyllid infectivity was associated with colonization of insect salivary glands by Lso and with Lso copy numbers >10,000 per psyllid. Results of our study indicate that Lso requires a two-week latent period in potato psyllids and suggest that acquisition and transmission of Lso by psyllids follows a pattern consistent with a propagative, circulative, and persistent mode of transmission.

Impact of Potato Planting Time on Incidence of Potato Zebra Chip Disease in the Lower Rio Grande Valley of Texas

Abstract. Zebra chip, a new and economically important disease of potato, Solanum tuberosum L., in the United States, Mexico, Guatemala, Honduras, Nicaragua, and New Zealand has caused millions of dollars in loss to the potato industry. The disease is associated with the bacterium “Candidatus Liberibacter solanacearum” transmitted to potato by the potato psyllid, Bactericera cockerelli (Sulc). Since its initial U.S. appearance in southern Texas in 2000, zebra chip has caused serious damage to potato production in the state, often leading to abandonment of entire fields. A study at Weslaco assessed the impact of potato planting time on incidence of zebra chip in the Lower Rio Grande Valley of Texas. Non-treated experimental potato plots were planted in mid-December to midFebruary for 4 years, and incidence of zebra chip was estimated at harvest each potato-growing season. Results showed that, in the Lower Rio Grande Valley, potatoes planted early were more affected by zebra chip than were those planted later, with infection rate ranging from 23.2 to 72.7, 20 to 44.6, and 5.4 to 31.2% in potato plots planted in December, January, and February, respectively. The reasons behind this differential in disease incidence are unknown; however, rate of infection by “Candidatus Liberibacter solanacearum” of psyllids colonizing potatoes in the region is suspected. Information from this research will help growers in southern Texas whose potatoes are affected by zebra chip to minimize losses caused by this damaging disease by timely planting of potatoes and appropriately protecting fields from colonization by the potato psyllid.