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Title: Association of "Candidatus Liberibacter solanacearum" with zebra chip disease of potato established by graft and psyllid transmission, electron microscopy and PCR

Author
item SECOR, GARY - NORTH DAKOTA UNIV., FARGO
item RIVERA, V - NORTH DAKOTA UNIV., FARGO
item ABAD, J - USDA, APHIS, BELTSVILLE
item Lee, Ing Ming
item CLOVER, G - AUCKLAND NEW ZEALAND
item LIEFTING, L - AUCKLAND NEW ZEALAND
item LI, X - CANADA
item DE BOER, S - CANADA

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/9/2009
Publication Date: 6/1/2009
Citation: Secor, G.A., Rivera, V.V., Abad, J.A., Lee, I., Clover, G.R., Liefting, L.W., Li, X., De Boer, S.H. 2009. Association of "Candidatus Liberibacter solanacearum" with zebra chip disease of potato established by graft and psyllid transmission, electron microscopy and PCR. Plant Disease. 93:574-583.

Interpretive Summary: Potato purple top wilt and similar diseases have caused tremendous damage to potato tuber production in South America, Mexico and the U.S. Earlier studies indicated that phytoplasmas, small cell wall-less bacteria, are the primary causal agents. During the last decade, a new potato disease with symptoms similar to those of potato purple top occurred in the southwestern United States, Mexico and Guatemala causing patchy brown discoloration of chips produced from commercial processing potatoes. This new disease, termed potato zebra chip, has caused severe economic losses in all market classes of potatoes. The cause of zebra chip has not been determined. In this study, we consistently detected a bacterium very closely related to the one named ‘Candidatus Liberibacter solanacearum’ in field samples of potato plants collected from several locations in the United States, Mexico, and Guatemala. Electron microscopic observation of diseased tissues showed the bacteria present in the plant. The results confirm the association of ‘Ca. Liberibacter solanacearum’ with zebra chip-affected potatoes. The information will be used by quarantine regulators, extension workers and plant diagnosticians to determine how to combat the disease.

Technical Abstract: A new disease of potatoes tentatively named zebra chip (ZC) because of the intermittent dark and light symptom pattern in affected tubers which is enhanced by frying, was first found in Mexico in 1994 and in the southwestern United States in 2000. The disease can cause severe economic losses in all market classes of potatoes. The cause of ZC has been elusive, and only recently has been associated with ‘Candidatus Liberibacter’ sp. Field samples of potato plants were collected from several locations in the United States, Mexico, and Guatemala to determine transmission to potato and tomato by grafting of ZC-infected scions and psyllid feeding. The disease was successfully transmitted, through up to three generations, by sequential top- and side-grafting ZC-infection scions to several potato cultivars and to tomato. The disease was also successfully transmitted to potato and tomato plants in greenhouse experiments by potato psyllids collected from potato plants naturally affected with ZC. Transmission electron microscopic observation of ZC-affected tissues revealed the presence of bacteria-like-organisms (BLOs) in the phloem of potato and tomato plants inoculated by grafting and psyllid feeding. The BLOs were morphologically similar in appearance to BLOs associated with other plant diseases. PCR amplified 16S rDNA sequences from samples representing different geographic areas including the United States, Mexico, and Guatemala, were almost identical to the 16S rDNA of ‘Ca. L. solanacearum’ previously reported from solanaceous plants in New Zealand and the United States. Two sub-clades were identified that differed in two single base-pair substitutions. These data confirm the association of ‘Ca. L. solanacearum’ with potatoes affected by ZC in the United States, Mexico, and Guatemala.