Jeremy L. Buchman

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.

Seasonal Occurrence and Abundance of the Potato Psyllid, Bactericera cockerelli, in South Central Washington

The potato psyllid, Bactericera cockerelli Sulc, has recently been identified as a vector of Candidatus Liberibacter solanacearum, the putative causal agent of zebra chip potato disease. Zebra chip is causing millions of dollars in losses to the potato industry in the United States, Mexico, Central America, and New Zealand. Currently, the most effective strategy to manage this potato disease is to target the potato psyllid with insecticides. Effective management of this insect pest requires knowledge of its biology, ecology, geographic distribution, and population dynamics. Although it is well documented that the potato psyllid is common throughout the western United States, several reports have indicated that this insect pest does not occur in Washington and Oregon. However, this insect has recently been observed and collected in this region. Studies were conducted from 2005 to 2008 to document and determine the seasonal occurrence of the potato psyllid in this important potato growing region of the United States. The potato psyllid was monitored in untreated experimental potato plots at Moxee and Prosser in south central Washington. Contrary to previous reports, the potato psyllid was found to occur in Washington and appears to migrate into the region late in the growing season. Upon arrival in south central Washington in late July, this insect readily reproduces in potatoes and appears to have at least one generation a year. The origin of potato psyllids migrating to Washington has not yet been determined. Information from this study will help potato growers in Washington manage the potato psyllid to better prevent potential zebra chip outbreaks.ResumenEl psílido de la papa, Bactericera cockerelli Sulc, se ha identificado recientemente como un vector de Candidatus Liberibacter solanacearum, el presunto agente causal de la enfermedad de la papa Zebra chip. Esta enfermedad esta causando millones de dólares en pérdidas en la industria de la papa en Estados Unidos, México, Centroamérica y Nueva Zelanda. Actualmente, la estrategia más efectiva para manejar esta enfermedad de la papa es enfocarse al psílido de la papa con insecticidas. El manejo efectivo de este insecto plaga requiere del conocimiento de su biología, ecología, distribución geográfica y dinámica poblacional. Aún cuando está bien documentado que el psílido de la papa es común a lo largo del oeste de los Estados Unidos, varios reportes indican que este insecto no se presenta en Washington y Oregon. No obstante, se le ha observado y colectado a este insecto en esta región. Se han hecho estudios de 2005 a 2008 para documentar y determinar la ocurrencia estacional del psílido de la papa en esta región tan importante de cultivo de papa en los Estados Unidos. El psílido de la papa se ha monitoreado en lotes experimentales de papa no tratados en Moxee y Prosser en la parte centro-sur de Washington. Contrario a reportes previos, se ha visto que el psílido de la papa se presenta en Washington y parece migrar al interior de la región al final del ciclo de cultivo. Al llegar al centro sur de Washington a finales de julio, el insecto se reproduce rápidamente en papa y parece tener por lo menos una generación al año. Aun no ha sido determinado el origen de los psílidos de papa que emigran a Washington. La información de este estudio ayudará a los productores de papa en Washington a manejar el psílido para una mejor prevención de establecimientos potenciales de zebra chip.

Effects of Liberibacter-Infective Bactericera cockerelli (Hemiptera: Triozidae) Density on Zebra Chip Potato Disease Incidence, Potato Yield, and Tuber Processing Quality

ABSTRACT 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 (Šulc), 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.

Candidatus Liberibacter solanacearum titer over time in Bactericera cockerelli (Hemiptera: Triozidae) after acquisition from infected potato and tomato plants.

ABSTRACT The potato psyllid, Bactericera cockerelli (Šule) (Hemiptera: Triozidae) is a serious pest of potato and other solanaceous crops. B. cockerelli has been associated with the bacterium “Candidatus Liberibacter solanacearum” (Lso), the causal agent of zebra chip, a new and economically important disease of potato in the United States, Mexico, Central America, and New Zealand. The biology of liberibacter transmission to potato and other host plants by the potato psyllid is largely unknown. The current study determined Lso acquisition by adult psyllids following different acquisition access periods (AAP) on potato and tomato, quantified Lso titer over time in postacquisition psyllids, determined Lso-acquisition rate in psyllids at each AAP on each source of inoculum, and determined influence of host plant Lso titer on Lso acquisition rates and postacquisition titer in psyllids over time. Results showed that Lso detection rates and titer increased over time in psyllids following AAPs of 8,24, and 72 h on tomato and potato and Lso titer was highest when psyllids acquired Lso from tomato versus potato. Lso titer ranged from 200- to 400-fold higher in tomato leaves, petioles, and stems than those of potato. The increase of Lso titer in the insects reached a plateau after an average of 15 d following 24 and 72 h AAP on potato or tomato. At this 15-d plateau, Lso titer in postacquisition psyllids was comparable with that of infective psyllids from the Lso-infected laboratory colony. Lso-acquisition rate in psyllids fed on potato and tomato increased up to 5 and 20, 15 and 35, 35 and 75, and 80 and 100%, respectively, when the insects were allowed access to plants for 4, 8, 24, and 72 h, respectively.

Incidence of the Beet Leafhopper-Transmitted Virescence Agent Phytoplasma in local Populations of the Beet Leafhopper, Circulifer tenellus, in Washington State

Abstract Phytoplasma diseases are increasingly becoming important in vegetable crops in the Pacific Northwest. Recently, growers in the Columbia Basin and Yakima Valley experienced serious outbreaks of potato purple top disease that caused significant yield loss and a reduction in tuber processing quality. It was determined that the beet leafhopper-transmitted virescence agent (BLTVA) phytoplasma was the causal agent of the disease in the area and that this pathogen was transmitted by the beet leafhopper, Circulifer tenellus Baker (Hemiptera: Cicadellidae). To provide the most effective management of phytoplasmas, timing of insecticide applications targeted against insects vectoring these pathogens should be correlated with both insect abundance and infectivity. Beet leafhoppers were collected from a potato field and nearby weeds in Washington throughout the 2005, 2006, and 2007 growing seasons and tested for BLTVA by PCR to determine the incidence of this phytoplasma in the insects. In addition, overwintering beet leafhoppers were collected throughout Columbia Basin and Yakima Valley and tested for BLTVA to investigate if these insects might constitute a source of inoculum for this phytoplasma from one season to the next. Results showed that 29.6% of overwintering leafhoppers collected near potato fields carried the phytoplasma. BLTVA-infected leafhoppers were also found in both potatoes and nearby weedy habitats throughout the growing season. PCR testing indicated that a large proportion of beet leafhoppers invading potatoes were infected with the phytoplasma, with an average of 20.8, 34.8, and 9.2% in 2005, 2006, and 2007, respectively. Similarly, BLTVA infection rate in leafhoppers collected from weeds in the vicinity of potatoes averaged 28.3, 24.5, and 5.6% in 2005, 2006, and 2007, respectively. Information from this study will help develop action thresholds for beet leafhopper control to reduce incidence of purple top disease in potatoes.

Impact of Zebra Chip Disease on the Mineral Content of Potato Tubers

Zebra chip (ZC) is an emerging and damaging disease of potato (Solanum tuberosum L.) that has been documented in potato fields throughout the southwestern United States, Mexico, Central America, and most recently New Zealand. The bacterium “Candidatus Liberibacter” has been putatively identified as the causal agent of this disease and the potato psyllid, Bactericera cockerelli (Sulc), as the insect vector. Tuber symptoms of ZC-infected plants include collapsed stolons, enlarged lenticels, vascular tissue browning, medullary ray discoloration, and necrotic flecking. ZC tuber symptoms are similar to those of potato diseases which exhibit perturbations in tuber mineral composition resulting in internal necrotic disorders. To determine the effect of ZC disease on the mineral content of potato tubers, concentrations of P, K, S, Ca, Mg, Na, Fe, Al, Mn, Cu, Zn, and B were analyzed in ‘Atlantic’ potatoes from ZC-infected and free plants, for both tuber flesh and peel tissues. The potatoes were grown at Weslaco, TX and Wapato, WA, two different and distant geographical locations under controlled cage conditions. Results showed that flesh tissue from ZC-infected tubers had significantly higher concentrations of P, K, and Ca at both locations, whereas, ZC-infected tuber flesh from Weslaco had higher concentrations of Mg, Cu, Zn, and B, while concentrations of Fe and Al were higher in ZC-infected tuber flesh from Wapato. Peel tissue from ZC-infected tubers were found to have a significantly higher level of Zn at both locations, whereas the levels of P and Na were found to significantly higher in peel tissue from Wapato and Weslaco, respectively. Finally, Mg was found to be significantly higher in ZC-free tubers from both locations and levels of Cu and B were significantly higher in ZC-free peel tissue from Wapato.ResumenZebra chip (ZC) es una enfermedad emergente y dañina de papa (Solanum tuberosum L.) que ha sido documentada en campos de papa en todo el sureste de los Estados Unidos, México, Centro América, y recientemente en Nueva Zelandia. Se ha identificado presuntamente a la bacteria “Candidatus liberibacter” como el agente causal de esta enfermedad, y al psílido de la papa Bactericera cockerelli (Sulc) como el insecto vector. Los síntomas del tubérculo de plantas infectadas con ZC incluyen el colapso de los estolones, lenticelas alargadas, oscurecimiento del tejido vascular, coloración radial medular y manchas necróticas. Los síntomas de ZC en tubérculo son similares a los de enfermedades de papa que muestran alteraciones en la composición mineral del tubérculo que resulta en desordenes internos necróticos. A fin de determinar el efecto de la enfermedad ZC en el contenido mineral de tubérculos de papa, se analizaron las concentraciones de P, K, S, Ca, Mg, Na, Fe, Al, Mn, Cu, Zn, y B, en plantas infectadas con ZC y sanas de la variedad Atlantic de papa, tanto de la pulpa como de la cutícula. Las papas se cultivaron en Weslaco, TX y Wapato, WA, dos ubicaciones geográficas diferentes y distantes, bajo condiciones controladas de confinamiento. Los resultados mostraron que el tejido de la pulpa de tubérculos infectados con ZC tenían significativamente mayores concentraciones de P, K, y Ca en ambas localidades, mientras que la pulpa de tubérculos infectados con ZC de Weslaco tenían mayores concentraciones de Mg, Cu, Zn, y B, y las concentraciones de Fe y Al fueron mas altas en pulpa de tubérculos infectados con ZC de Wapato. Se encontró que el tejido de la cutícula de tubérculos infectados con ZC tenía un nivel significativamente más alto de Zn en ambas localidades, mientras que P y Na fue mayor con significancia en tejido cuticular de Wapato y Weslaco, respectivamente. Finalmente, se encontró que el Mg fue significativamente mayor en tubérculos libres de ZC de ambas localidades, y los niveles de Cu y B fueron significativamente más altos en tejido cuticular libre de ZC de Wapato.

Susceptibility of Different Potato Cultivars to Purple Top Disease

In 2002, Columbia Basin potato growers in Washington and Oregon began to experience serious outbreaks of potato purple top disease that have caused significant yield losses and reductions in tuber quality. It was determined that the beet leafhopper-transmitted virescence agent (BLTVA) phytoplasma is the causal agent of the disease in the Columbia Basin and that this pathogen is transmitted by the beet leafhopper. Little is known about the impact of purple top disease on potato in the Pacific Northwest. Trials were conducted in 2006, 2007, and 2008 to assess susceptibility of important potato cultivars grown in the Columbia Basin to purple top under field conditions. Eight potato cultivars were planted in small field plots and exposed to naturally occurring BLTVA-infected beet leafhoppers. Disease incidence was recorded at the end of the growing season. Purple top foliar symptoms were observed in all the tested cultivars, and plant samples collected and tested by PCR confirmed the presence of BLTVA. Results also indicated significant differences in susceptibility of the cultivars to purple top. Disease incidence in leafhopper-exposed plots ranged from 1.5 to 49.3% among cultivars. Russet Norkotah, Ranger Russet, and Umatilla Russet were relatively high in susceptibility to purple top, with average disease incidence of 33.5, 23.3, and 19.4%, respectively. Russet Burbank and FL1967 showed some resistance to purple top as they had the lowest plant disease incidence of 2.3 and 4.4%, respectively. Alturas, Shepody, and FL 1879 were moderately susceptible to purple top, with average disease incidence of 12.8, 8.0, and 9.3%, respectively. Information from this study will help potato growers reduce damage caused by potato purple top through selection of cultivars that are resistant to or tolerant of the disease and by protecting susceptible varieties against the beet leafhopper.ResumenEn 2002, productores de papa de la ribera del Columbia en Washington y Oregon empezaron a experimentar serias incidencias de la enfermedad de la punta morada de la papa que ha causado significativas pérdidas en rendimiento y reducción en la calidad de tubérculo. Se determinó que el fitoplasma de la virescencia (BLTVA) transmitido por la chicharrita de la remolacha es el agente causal de la enfermedad en la ribera del Columbia, y que este patógeno es transmitido por la chicharrita de la remolacha. Se sabe poco del impacto de la enfermedad de la punta morada en papa en el noroeste del pacífico. Se efectuaron ensayos en 2006, 2007 y 2008 para evaluar la susceptibilidad bajo condiciones de campo a la punta morada, de las variedades importantes de papa que se cultivan en la ribera del Columbia. Se plantaron ocho cultivares en pequeños lotes de campo y se expusieron a la presencia natural de chicharritas de la remolacha infectadas con BLTVA. Se registró la incidencia de la enfermedad al final del ciclo de cultivo. Se observaron síntomas foliares de la punta morada en todas las variedades probadas, y muestras de plantas colectadas y probadas por PCR confirmaron la presencia de BLTVA. Los resultados también indicaron diferencias significativas en susceptibilidad de las variedades a la punta morada. La incidencia de la enfermedad en los lotes expuestos a las chicharritas fluctuó de 1.5 a 49.3% entre variedades. Russet Norkotah, Ranger Russet y Umatilla Russet fueron relativamente altas en susceptibilidad a la punta morada, con un promedio de incidencia de la enfermedad de 33.5, 23.3 y 19.4, respectivamente. Russet Burbank y FL1967 mostraron alguna resistencia a la punta morada al tener la menor incidencia de la enfermedad de 2.3 a 4.4%, respectivamente. Alturas, Shepody y FL1879 fueron moderadamente susceptibles a la punta morada, con promedios de incidencia de la enfermedad de 12.8, 8.0 y 9.3%, respectivamente. La información de este estudio ayudará a los productores a reducir el daño causado por la punta morada mediante la selección de variedades que son resistentes o tolerantes a la enfermedad y con la protección de las variedades susceptibles contra la chicharrita de la remolacha.

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.