Studies on the possible transmission of ‘Candidatus Liberibacter solanacearum’ by carrot seeds
Trials have been conducted in France to study the transmission of ‘Candidatus Liberibacter solanacearum’ (potato haplotypes are listed on the EPPO A1 List) by seeds of carrot (Daucus carota). Earlier studies done by two independent laboratories had obtained contradictory results. One study (Loiseau et al., 2017) had not obtain any seed transmission, whereas the other one (Bertolini et al., 2015) had shown some transmission from carrot seeds to plants after 4 months of culture with a transmission rate ranging from 12 to 42%. In the current study, transmission trials were conducted in 2015 on 4 lots of 500 carrot seeds naturally contaminated with ‘Ca. L. solanacearum’ and on 2 lots of 100 healthy seeds. The contamination rate of the seed lots ranged from 96 to 100% (a higher rate than in the two previous studies). Plants were grown for 6 months in an insect-proof greenhouse. Each month, groups of 108 plants obtained from contaminated seed lots, as well as groups of 24 plants obtained from healthy seeds, were tested by real-time PCR for ‘Ca. L. solanacearum’. Testing of seeds and plants from healthy lots always gave negative results. During the 6 months of the trial, no plant obtained from contaminated seeds tested positive or showed any symptoms. These results and the results of the previous studies suggest that transmission of ‘Ca. L. solanacearum’ by carrot seeds is rare and difficult to reproduce. It is concluded that taking into account the results of the present study and of the observed geographical distribution of the disease on carrot crops, seeds are not a major transmission pathway. Although more studies are needed, psyllids (Trioza apicalis and Bactericera trigonica) probably play a more efficient role in transmitting the bacterium. The presence of cultivated or weed plants capable of acting as reservoirs in the vicinity of carrot crops should also be further studied.
Sources
Loiseau M, Renaudin I, Cousseau-Suhard P, Lucas PM, Forveille A, Gentit P (2017) Lack of evidence of vertical transmission of ‘Candidatus Liberibacter solanacearum’ by carrot seeds suggests that seed is not a major transmission pathway. Plant Disease 101(12), 2104-2109.
Earlier studies
Bertolini E, Teresani GR, Loiseau M, Tanaka FAO, Barbé S, Martínez C, Gentit P, López MM, Cambra M (2015) Transmission of ‘Candidatus Liberibacter solanacearum’ in carrot seeds. Plant Pathology 64(2), 276-285.
Loiseau M, Renaudin I, Cousseau-Suhard P, Poliakoff F, Gentit P (2017) Transmission tests of ‘Candidatus Liberibacter solanacearum’ by carrot seeds. Acta Horticulturae 1153, 41-46.