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Characterizing Zebra Chip Symptom Severity and Identifying Spectral Signatures Associated with ‘Candidatus Liberibacter solanacearum’-Infected Potato Tubers

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Zebra chip (ZC) disease has been a threat to the US potato industry for nearly two decades. ZC is associated with the bacteria ‘Candidatus Liberibacter solanacearum’ (Lso), which is transmitted by the potato psyllid, Bactericera cockerelli (Šulc) (Hemiptera: Triozidae). ZC reduces yield and quality, due to discoloration of the tuber tissue. Symptom severity is affected by the time of infection and early-season infections express relatively higher degrees of ZC. Although tubers infected late in the growing season, i.e. within 2 weeks of harvest, express minimal to no symptoms, they may proceed to express ZC symptoms following storage. Currently, visualization of ZC symptoms in tubers is used by processors to estimate the percentage of ZC-affected tubers in truckloads of potatoes. This approach, however, is time consuming and relies on evaluations of a small sample size. Thus, it is likely to fail in detecting the asymptomatic late infections. Using several potato genotypes infected at different times during the growing season, this study was set to determine if visible and near infrared spectrometry, and infrared thermal imaging, can be used to distinguish ZC-affected tubers, and to predict the severity of ZC symptoms. The subjective symptom score categorization, commonly used in ZC studies, corresponded with the percentage of the symptomatic area in the tubers sliced at the solon attachment end. The percentage of symptomatic area was also correlated with Lso titer. Reflectance effectively distinguished infected and uninfected tubers with high accuracy; the combined wavelengths 468, 582, 680 and 720-nm were the most effective in predicting symptom severity. Near infrared (NIR) and infrared (IR) thermal imaging failed to distinguish tubers based on Lso infection status after storage. The quantitative measure of ZC severity, and the effectiveness of some visual wavelengths in detecting ZC may help to facilitate selection assays for ZC resistance. While in our study infrared imaging did not appear to be effective in distinguishing Lso- affected tubers after storage, additional studies with field harvested tubers are still needed to evaluate the effectiveness of VIS, NIR and IR spectrometry in screening for ZC.


La enfermedad de la papa rayada (ZC, zebra chip, por sus siglas en inglés) ha sido una amenaza para la industria de la papa en los EU por cerca de dos décadas. Se asocia ZC con la bacteria ‘Candidatus Liberibacter solanacearum’ (Lso), que se transmite por el psílido de la papa Bactericera cockerelli (Šulc) (Hemiptera: Triozidae). ZC reduce el rendimiento y la calidad, debido a la pigmentación del tejido del tubérculo. Se afecta la severidad del síntoma por el tiempo de infección, y las infecciones tempranas expresan relativamente mayores grados de ZC. Aun cuando los tubérculos infectados tarde durante el ciclo del cultivo, i. e. dentro de las dos semanas a la cosecha, no expresan síntomas o son mínimos, se pudiera proceder a la expresión en el almacenamiento posterior. Actualmente, la visualización de los síntomas de ZC en tubérculos se usa por los procesadores para estimar el porcentaje de tubérculos afectados por ZC en cargamentos de papas. Este enfoque, no obstante, es tardado y confía en evaluaciones de un pequeño tamaño de muestra. De aquí que es muy probable que se falle en la detección de infecciones tardías asintomáticas. Mediante el uso de varios genotipos de papa infectados a diferentes tiempos durante el ciclo de cultivo, se estableció este estudio para determinar si la espectrometría visible e infrarroja cercana, y la imagen infrarroja térmica, pueden usarse para distinguir los tubérculos afectados, y para predecir la severidad de los síntomas por ZC. La categorización de la evaluación subjetiva del síntoma comúnmente usada en estudios de ZC, correspondió con el porcentaje del área sintomática en los tubérculos rebanados en el extremo de la unión con el estolón. El porcentaje del área sintomática también estuvo correlacionada con el título Lso. La reflectancia distinguió efectivamente los tubérculos infectados y no infectados con alta precisión; las longitudes de onda combinadas de 468, 582, 680, y 720 nm fueron las más efectivas en la predicción de la severidad del síntoma. El infrarrojo cercano (NIR) y la imagen de infrarrojo térmico fallaron para distinguir los tubérculos con base en el estatus de la infección Lso después del almacenamiento. La medición cuantitativa de severidad por ZC, y la efectividad de algunas longitudes de onda visuales en la detección de ZC pudieran ayudar a facilitar los ensayos de selección para resistencia a ZC. Mientras que en nuestro estudio la imagen infrarroja no pareció ser efectiva en la distinción del Lso de tubérculos afectados después del almacenamiento, aun se necesitan estudios adicionales con tubérculos cosechados en el campo para evaluar la efectividad de la espectrometría VIS, NIR, e IR, en los estudios para ZC.

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We would like to thank our laboratory technicians, Fabiola Aguilar, for her help with different aspects of this project, and Heather Headrick, for infrared thermal camera and software instructions. We would also like to thank Dr. Richard Novy for providing potato genotypes, and Dr. Nora Olsen and Lynn Woodell for facilitating the storage process for the tubers used in this study. This work was supported by the USDA-National Institute of Food and Agriculture, Hatch/Rashed project IDA01506 and USDA- National Institute of Food and Agriculture project 2014-67014-22408. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.

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Correspondence to Sean M. Prager or Arash Rashed.

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Zhao, Z., Prager, S.M., Cruzado, R.K. et al. Characterizing Zebra Chip Symptom Severity and Identifying Spectral Signatures Associated with ‘Candidatus Liberibacter solanacearum’-Infected Potato Tubers. Am. J. Potato Res. 95, 584–596 (2018).

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