Plant Science

‘Candidatus Liberibacter solanacearum’ is a bacterium associated with several vegetative disorders on solanaceous and apiaceous crops. Following the recent detection of the bacterium in carrots in Europe, and particularly carrot plants used for seed production in France, two independent laboratories conducted experiments on the transmission of this pathogen by seed and had discordant results: one study showed no bacterial transmission to plants, and the other showed transmission to carrot seedlings starting from the fourth month of culture. To test the hypothesis that growing conditions affect seed transmission efficiencies, trials were renewed in 2015 on four lots of 500 carrot seeds naturally contaminated with ‘Ca. L. solanacearum’ and two lots of 100 healthy seeds. The plants were grown for 6 months in an insect-proof NS2 greenhouse. Sets of 108 plants from the contaminated lots and 24 plants from the healthy lots were individually analyzed each month using real-time PCR to detect the bacterium. The detection tests on seeds and plants from healthy lots were always negative. During the 6 months of the trial, no plants from the contaminated seed lots tested positive for the bacterium or showed any infection symptoms. These results indicate that transmission of ‘Ca. L. solanacearum’ by carrot seed is rare and difficult to reproduce.

The use of proper management strategies for citrus huanglongbing ( HLB ), caused by ‘ Candidatus Liberibacter asiaticus’ (Las) and transmitted by Asian citrus psyllid ( ACP ) ( Diaphorina citri ), is a priority issue. HLB control is based on healthy seedlings, tolerant rootstock cultivars and reduction of ACP populations. Here, dynamic populations of Las in different citrus hosts and each instar of ACP were studied, together with the seasonal growth and distribution of Las in different tissues, using conventional and TaqMan real‐time PCR . Different levels of susceptibility/tolerance to HLB were seen, resulting in different degrees of symptom severity and growth effects on hosts or rootstocks. Troyer citrange, Swingle citrumelo and wood apple were highly tolerant among 11 rootstock cultivars. Regarding distribution and seasonal analysis of Las, mature and old leaves contained high concentrations in cool temperatures in autumn and spring. Las was detected earlier through psyllid transmission than through graft inoculation, and the amounts of Las ( AOL ) varied in different hosts. Thus, different AOL (10 4 –10 7 copy numbers μL −1 ) and Las‐carrying percentages ( LCP ; 40–53.3%) were observed in each citrus cultivar and on psyllids, respectively. Furthermore, both AOL and LCP were lower in nymphs than in adult psyllids, whereas the LCP of psyllids were not affected by increasing the acquisition‐access time. The present study has significant implications for disease ecology. The combination of early detection, use of suitable rootstocks and constraint of psyllid populations could achieve better management of HLB disease.