Resumen de Vector mediated transmission of Candidatus Liberibacter solanacearum and epidemiological consequences
Carlos Andrés Antolínez Delgado
Candidatus Liberibacter solanacerum (Lso) is an emerging pathogenic plant bacterium causing major economic losses in vegetable crops worldwide. Lso is transmitted in a circulative-propagative manner by different psyllid species (Hemiptera : Psylloidea). In Europe Lso causes economically important losses in carrot (Daucus carota L.) and celery (Apium graveolens L.) crops and has become a potential threat to the potato (Solanum tuberosum L.) industry. This Thesis focuses on studying the vector transmission of Lso by psyllids and its epidemiological implications.
The first objective of this Thesis was to study the feeding behavior of the main Lso vector Bactericera trigonica Hodkinson in the Mediterranean region, by characterizing its feeding behavior on carrot seedlings by means of the Electrical Penetration Graph (EPG) technique. Waveforms identified were correlated with stylet tracks and salivary sheath termini on plant tissues as well as with Lso inoculation. Also, the effect of Lso in the feeding behaviour of B. trigonica was tested by EPGs. Seven different waveforms (C1, C2, pseudo-potential drop, G, D, E1 and E2) were identified and correlated with salivary sheath termini in mesophyll (C1, C2 and Pseudo-potential drop), xylem (G) and phloem tissues (D, E1 and E2). “Pseudo-potential drop” is a novel waveform pattern never previously described for psyllids. Here, the first and full description of this waveform is provided and its biological meaning discussed. This study showed that B. trigonica is a highly efficient vector of Lso and that there is a direct correlation between waveform E1 and the inoculation of Lso. Additionally, I found that Lso directly affects the feeding behaviour of B. trigonica, but has no influence in its own transmission (the pathogen does not enhance or impair transmission).
The second objective of this study focused on how vector sex affected the feeding behaviour and consequently the transmission of Lso. By using EPGs the feeding behaviour of males and females of B. trigonica on carrots was monitored and correlated with Lso transmission rate by single or groups of males and females of B. trigonica. In addition, the titre of Lso transmitted by males or females as well as the symptoms expression produced by Lso on carrots was quantified. The sex of the vector affected the feeding behaviour and Lso titre. Females probed and ingested more from the phloem tissues than males and this was correlated with more Lso titter inoculated by females than by males. Despite Lso titter differences, transmission rate was not affected by sex and both males and females transmitted Lso very efficiently. This study also showed that groups of females more severe symptoms than groups of males, however this difference was not evident when comparing symptoms produced by single males or single females. These results show a high vector efficiency of B. trigonica and highlight the importance of vector sex in the transmission of Lso.
Third, the risk of transmission of Lso from carrots to other economically important crops was assessed. In the Mediterranean region, B. trigonica is the only known vector of Lso, but the leek-onion psyllid Bactericera tremblayi Wagner is another widespread psyllid and potential vector of Lso. Thus, the settling, oviposition and feeding behaviour of B. trigonica and B. tremblayi in potato, carrot, celery and leek were studied and the likely implications for the spread of Lso were determined. Bactericera trigonica was unable to transmit Lso efficiently to potato nor to colonize potato. Additionally, B. trigonica showed a clear preference to ingest sap from the phloem, settle and oviposit on carrot and celery. Consequently, Lso was efficiently transmitted to both carrot and celery. On the other hand B. tremblayi preferred leek over carrot and potato, with potato being the less preferred host. Additionally B. tremblayi was able to ingest from the phloem of carrot and consequently could acquire Lso but failed to transmit the bacterium from carrot to carrot. Results showed that despite B. trigonica can transmit Lso to potato with very low efficiency it is not a threat to potato crops. Furthermore, B. tremblayi is not a likely vector of Lso.
Finally, population densities of psyllid associated to commercial carrot and potato fields were estimated in 2015 and 2016 by three different sampling methods (sweep net, green tile traps and visual inspection of eggs and nymphs). Commercial plots were surveyed in two of the most important carrot producing regions in Spain: Castilla-Leon and Comunidad Valenciana, more preciselyin Gomezserracín (Segovia), Ìscar (Valladolid) and Villena (Alicante). In addition, psyllid populations in two commercial potato plots Aldearrubia (Salamanca) and Gomezserrcìn (Segovia) were monitored. B. trigonica was the most abundant psyllid in all carrot fields followed by B. nigricornis. On the other hand, B. nigricornis was the most abundant psyllid species associated with potato fields in both 2015 and 2016 and was the only psyllid species able to complete its life cycle in potato plants. B. trigonica was associated with one potato plot that was planted next to a carrot infested crop. Data suggests that B. trigonica can colonize carrot but not potato and that B. nigricornis is able to colonize both carrot and potato.
This Thesis provides new information on the feeding behaviour, the transmission ability and the population dynamics of psyllids on Apiaceae and potato crops. Such information is critical for understanding the epidemiology of Lso and for developing effective disease management strategies.
