Candidatus Liberibacter solanacearum (CaLso) is a phloem Gram-negative limited bacterium causing the “Zebra chip” (ZC) in potato and other vegetative disorders in other Solanaceae crops (i.e. tomatoes, peppers, tamarillos) in Central, North America and New Zealand. Interestingly the bacterium has been detected in Finland, Sweden, Germany, Norway, and Canary Islands and in the mainland Spain causing disease in carrots and celery commercial crops.
CaLso is transmitted from potato mother tubers to growing plants and to progeny tubers. Moreover, CaLso is naturally spread by different species of psyllids (Hemiptera: Triozidae), a group of phloem feeders that have been always correlated with leaf yellows and yield crop reduction. While the American potato psyllid, Bactericera cockerelli, is the known vector of CaLso in solanaceous plants in North America and New Zealand, two different psyllid species have been reported so far as vectors of CaLso in Apiaceae crops: Trioza apicalis in northern Europe and Bactericera trigonica in the Mediterranean region.
Five different haplotypes of the bacterium have been described (designated in order of description as A, B, C, D and E) according to their ability to infect different crops in specific geographical regions. CaLso haplotypes A and B occur on Solanaceae plants in America and New Zealand and CaLso haplotypes C, D, E occur on members of the Apiaceae family in northern Europe and the Mediterranean regions.
It is known that CaLso is transmitted in a propagative-circulative manner (it needs to replicate in the vector body before inoculation) by vectors and that vector salivation and vector sap ingestion into the phloem sieve tubes are an absolute prerequisite for pathogen transmission.
Due to its relative recent emergence, there is a lack of information on aspects related to the transmission, vector-bacteria interactions and the epidemiology of CaLso. For example, there are few studies on the transmission characteristics of CaLso by their psyllid vectors such as B. cockerelli, B. trigonica and T. apicalis as well as other psyllid species that may also act as vectors.
For example, is there any specificity for the transmission of the different bacterial haplotypes transmission by the different vector species? Is it possible the transmission of several bacterial haplotypes by different vectors between different hosts?
It is obvious that the haplotypes present in a given area (i.e. A and B in North America) are transmitted by the psyllid species that is dominant in the region (i.e. B. cockerellii). However, this fact is not indicative that there is a vector-haplotype specificity in CaLso transmission, because there might be no other potential vectors colonizing the crop on the given region of study. What would happen if a psyllid species that feeds and colonize solanaceaus crops (eg. Russelliana solanicola) is introduced into one of the potato growing areas of Oregon State in the United States (i.e)? Until now, there is not information about bacterial-vector specificity, and in my opinion, there is still much to investigate on the relationship of CaLso-vector interactions and the epidemiology of vector-borne bacterial diseases.
It is known that in the field, phloem-restricted pathogens are transmitted efficiently only by colonizing species and, from an epidemiological point of view, the transmission of a persistent phloem-restricted plant pathogen by non-colonizing species is very unlikely to occur in the field. That fact would explain why B. cockerellii is the main vector of CaLso in potato in North America or B. trigonica is the main vector in carrots in Spain. However, this does not exclude the possibility that other psyllid species sporadically probe on a non-host crop, particularly if that crop is the only available in the field at a given time of the year. The rate of plant pathogen transmission would be lower than expected but this psyllid species could be an occasional vector to crops other than its main host. Probably this psyllid species would not be involved in the secondary spread of the pathogen but it would be responsible to its primary spread acquiring a main role on the bacterium epidemiology.
So, if the previous assumption would be possible, could a given bacterial haplotype able to infect Apiaceae crops become a serious problem in Solanaceae crops (i.e)? That would be one question that could be considered in areas where potato crops are growing close to carrot crops, for example. Would the infected carrots act as a primary inoculum source of the bacterium? Could other weeds and natural vegetation near to the crops be the potential reservoirs CaLso? Would a given psyllid species colonizing carrots (i.e. B. trigonica) be responsible of an occasional transmission to potatoes? Or is there any other psyllid species able to feed sporadically in carrots and then feed on potato transmitting the bacteria?
Obviously, the first step to solve this enigma would be to verify if the same haplotype is able to inoculate both crops (eg carrot and potatoes) . Once this is known, the second thing to do would be to identify the potential vectors of the bacterium.
Take as an example the situation in Spain (because it is the most close to me). Both crops, Solanaceae and Apiaceae, can share growing regions simultaneously for some time periods, so it would be possible that this theoretical situation occurs. The most abundant psyllid species found in carrots in Spain is B. trigonica that is the main vector of CaLso in carrots. However, B. tremblayi and B.nigricornis have been found in carrots, celery or potato crops. Until now, it is not yet clear if B. trigonica is able to transmit the bacterium to other crops or if these other psyllid species could be involved in the spread of CaLso in the field.
In any case, if this unknown psyllid species is able to feed on different crops it could become a “potential vector” of CaLso on different crops. For example, if viruliferous psyllids can fly from Apiaceae-infected hosts to solanaceous crops (or viceversa) and then land and feed for a short time they would move the primary inoculum to an unexpected host plant. This is an example of why it is mandatory to improve our knowledge on this epidemiological aspect in order to prevent or reduce the spread of the bacterial under field conditions.
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