‘Candidatus Liberibacter psyllaurous’, a new species causing diseases of solanaceous crops: addition to the EPPO Alert List
Studies which have been carried out in the USA and New Zealand almost simultaneously came to the conclusions that a new bacterium species belonging to the genus ‘Candidatus Liberibacter’ was associated with emerging diseases of potatoes, tomatoes and other solananacous crops, and that this fastidious and phloem-inhabiting bacterium was transmitted by a psyllid vector, Bactericera cockerelli (syn. Paratrioza cockerelli, Hemiptera: Psyllidae). In addition, it has been observed in North America that B. cokerelli populations were often associated with a growth disorder in potato and tomato called ‘psyllid yellows’. For many years, it has been considered that this disorder was caused by a toxin produced by the psyllid but it is now felt that this disorder is most probably caused by the new ‘Candidatus Liberibacter’ species.
Situation in the Americas
Since the early 1990s, a new disease of potatoes (Solanum tuberosum) called ‘zebra chip’ (or ‘papa manchada’ in Spanish) has been reported in the Americas (e.g. Guatemala, Mexico and the Southwestern USA). The disease caused severe economic losses, in particular to the potato chip industry because chips made from infected tubers present dark stripes becoming markedly more visible after frying which is unacceptable for manufacturers. When planted, infected tubers do not produce plants in some cases. Observations made in affected potato fields strongly suggested that the disease was transmitted by the potato/tomato psyllid, B. cockerelli.
Situation in New Zealand
In New Zealand, a new disease of glasshouse tomatoes (Lycopersicon esculentum) was observed in Auckland in January 2008. Affected plants showed spiky and chlorotic apical growth, leaf curling, mottling, and general stunting. B. cockerelli was reported to occur in all tomato glasshouses where the disease was observed. In New Zealand, B. cockerelli is an introduced pest which was first discovered in May 2006 in 1 glasshouse in Auckland. Today, it is established throughout the North Island and the northern part of the South Island. In April 2008, similar symptoms appeared in a glasshouse of Capsicum annuum on the same property. In May 2008, symptoms resembling ‘zebra chip’ disease were observed in potatoes (tubers had been harvested from a breeding trial in South Auckland). The affected tubers showed necrotic flecking and streaking which became marked when the potatoes were fried. Diseased plants generally senesced early (beginning of April in New Zealand), yield was reduced (60 % less than expected) and harvested tubers had less dry matter (13% instead of 19%). Large numbers of B. cockerelli were observed on the diseased crops.
Results of the US and New Zealand studies
Investigations carried out in parallel in the USA and New Zealand revealed the presence of bacterium-like organisms in diseased solanaceous crops which were called ‘Candidatus Liberibacter psyllaurous’ (Hansen et al., 2008) and ‘Candidatus Liberibacter solanacearum’ (Liefting et al., 2009), respectively. Apparently, these two new pathogens are closely related, if not identical (e.g. comparison studies of rDNA sequences revealed 99.7% identity between potato isolates from Texas with potato and tomato isolates from New Zealand). For the moment, the EPPO Secretariat has considered that these two pathogens were identical, and because ‘Candidatus Liberibacter psyllaurous’ was the first name proposed, it was retained as the preferred name. Considering the importance of solanacous crops in the EPPO region, the EPPO Secretariat decided to add ‘Candidatus Liberibacter psyllaurous’ to the EPPO Alert List.
‘Candidatus Liberibacter psyllaurous’
Why: A new bacterial species ‘Candidatus Liberibacter psyllaurous’ has been found in association with serious diseases of tomatoes, potatoes and other solanaceous crops observed in the Americas, and recently discovered in New Zealand. In particular, it was found associated with a potato disease called ‘zebra chip’ which has caused significant economic losses, by reducing both yield and quality of potato crops. The tomato/potato psyllid Bactericera cockerelli (syn. Paratrioza cockerelli, Hemiptera: Psyllidae) is strongly suspected to be the vector of this new bacterium.
Where: The geographical distribution given below is essentially based on reports of disease symptoms. However, the presence of ‘Ca. L. psyllaurous’ has been confirmed in New Zealand (at first under another tentative name ‘Ca. L. solanacearum’), in Mexico (Coahuila), and the USA (California, Kansas, Texas). As molecular tools are now available to detect specifically ‘Ca. L. psyllaurous’, further studies will probably better determine its geographical distribution.
EPPO region: Absent.
North America: Canada (Alberta), Mexico, USA (Arizona, California, Colorado, Idaho, Kansas, Montana, Nebraska, Nevada, New Mexico, North Dakota, Texas, Utah, and Wyoming).
Central America: Guatemala, Honduras.
Oceania: New Zealand.
The distribution of the psyllid vector, B. cokerelli is the following:
EPPO region: Absent.
North America: Canada (Alberta, British Columbia, Ontario, Quebec, Saskatchewan), Mexico, USA (Arizona, California, Colorado, Idaho, Kansas, Minnesota, Montana, Nebraska, Nevada, New Mexico, North Dakota, Oklahoma, South Dakota, Texas, Utah, Wyoming).
Central America: Guatemala, Honduras.
Oceania: New Zealand (recently introduced, first detected in May 2006 in Auckland).
On which plants: Potato (Solanum tuberosum), tomato (Lycopersicon esculentum), and capsicum (Capsicum annuum). The presence of ‘Ca. L. psyllaurous’ was detected in symptomless Solanum betaceum (tamarillo) and Physalis peruviana (Cape gooseberry). These plants were collected from a garden in South Auckland, located close to a commercial glasshouse where infected tomatoes had been found. For the moment, it is not known whether S. betaceum and P. peruviana only act as symptomless reservoirs of the pathogen or can also develop disease symptoms. Although it can be found on many plants (numerous species in 20 plant families), the psyllid vector (B. cockerelli) has been reported to complete its life cycle only on Solanaceae, Convolvulaceae and Lamiaceae. Its preferred hosts include aubergine, capsicum tomato, and potato.
Damage: On potato symptoms include: purple top, shortened internodes, smaller leaves, enlargement of the stems, swollen axillary buds and aerial tubers. Potato chips made from infected tubers present dark stripes which become markedly more visible after frying (hence the disease name ‘Zebra chip’), leading to rejection from the potato chip industry. When planted, infected tubers may not produce plants. ‘Zebra chip’ disease has been reported to cause severe economic losses in potato production (up to 60 % yield losses and significant rejections from the industry). Although significant economic damage has been reported on potato crops in Guatemala, Mexico and the Southwestern USA, the economic impact of the disease in New Zealand is yet to be determined.
On tomato symptoms include: ‘spiky’ and chlorotic apical growth, leaf curling, mottling, plant stunting, and in some cultivars fruit deformation.
On capsicum affected plants develop: chlorotic or pale green leaves, sharp tapering of leaf apex, upward leaf curling, shortened internodes and petioles, necrosis of apical meristem, flower abortion, and plant stunting.
Transmission: Preliminary transmission trials strongly suggested that B. cokerelli is a vector of ‘Ca. L. psyllaurous’. It has been demonstrated that the psyllid can acquire the bacterium but transmission needs to be confirmed. In addition, many other aspects of the disease epidemiology remain to be studied (e.g. transmission through seeds or grafts). Over long distances, trade of infected plants and psyllids can spread the bacterium.
Pathway: Plants for planting of host plants, tomato and capsicum fruits? potato tubers? seeds? infected psyllids?
Possible risks: Solanaceous crops such as potatoes and tomatoes are extensively grown in the EPPO region and are of major economic importance. For the moment, there is little experience with disease control, and it is likely that it will be essentially targeted against the psyllid vector (or possibly the use of resistant cultivars). Although many aspects of the biology and epidemiology of ‘Ca. L. psyllaurous’ need to be further investigated, it is advisable to avoid its introduction into the EPPO region, as well as of its psyllid vector, B. cockerelli.
EPPO RS 2009/089
Panel review date: -
Entry date 2009-05
Abad JA, Bandhla M, French-Monar RD, Liefting LW, Clover GRG (2008) First report of the detection of ‘Candidatus Liberibacter’ species in Zebra chip disease-infected potato plants in the United States. Plant Disease 93(1), p 108.
Biosecurity Australia (2009) Draft pest risk analysis report for ‘Candidatus Liberibacter psyllaurous’ in fresh fruit, potato tubers, nursery stock and its vector the tomato-potato psyllid. Biosecurity Australia, Canberra, 110 pp. http://www.daff.gov.au/__data/assets/pdf_file/0008/1108691/Candidatus_Liberibacter_psyllaurous_draft_PRA_20090506.pdf
Crosslin JM , Bester G (2009) First report of ‘Candidatus Liberibacter psyllaurous’ in Zebra Chip symptomatic potatoes from California. Plant Disease 93(5), p 551.
Hansen AK, Trumble JT, Stouthamer R, Paine TD (2008) A new huanglongbing species, ‘Candidatus Liberibacter psyllaurous,’ found to infect tomato and potato, is vectored by the psyllid Bactericera cockerelli (Sulc). Applied and Environmental Microbiology 74(18), 5862-5865.
Liefting LW, Sutherland PW, Ward IL, Paice KL, Weir BS, Clover GRG (2009) A new ‘Candidatus Liberibacter’ species associated with diseases of Solanaceous crops. Plant Disease 93(3), 208-214.
Liefting LW, Perez-Egusquiza, Clover GRG, Anderson JAD (2008) A new ‘Candidatus Liberibacter’ species in Solanum tuberosum in New Zealand. Plant Disease 92(10), p 1474.
Liefting LW, Ward LI, Shiller JB, Clover GRG (2008) A new ‘Candidatus Liberibacter’ species in Solanum betaceum (tamarillo) and Physalis peruviana (Cape gooseberry) in New Zealand. Plant Disease 92(11), p 1588.
Munyaneza JE, Sengoda VG, Crosslin JM, de la Rosa-Lozano G, Sanchez A (2009) First report of ‘Candidatus Liberibacter psyllaurous’ in potato tubers with Zebra Chip disease in Mexico. Plant Disease 93(5), p 552.