Transmission of ‘Candidatus Liberibacter solanacearum’ in carrot seeds

Abstract

A protocol for the specific detection and quantification of ‘Candidatus Liberibacter solanacearum’ in carrot seeds using real‐time PCR was developed. The bacterium was detected in 23 out of 54 carrot seed lots from 2010 to 2014, including seeds collected from diseased mother plants. The average total number of ‘Ca. L. solanacearum’ cells in individual seeds ranged from 4·8 ± 3·3 to 210 ± 6·7 cells per seed from three seed lots, but using propidium monoazide to target live cells, 95% of the cells in one seed lot were found to be dead. Liberibacter‐like cells were observed in the phloem sieve tubes of the seed coat and in the phloem of carrot leaf midrib from seedlings. The bacterium was detected as early as 30 days post‐germination, but more consistently after 90 days, in seedlings grown from PCR positive seed lots in an insect‐proof P2 level containment greenhouse. Between 12% and 42% of the seedlings from positive seed lots tested positive for ‘Ca. L. solanacearum’. After 150 days, symptoms of proliferation were observed in 12% of seedlings of cv. Maestro. ‘Candidatus Liberibacter solanacearum’ haplotype E was identified in the seeds and seedlings of cv. Maestro. No phytoplasmas were detected in seedlings with symptoms using a real‐time assay for universal detection of phytoplasmas. The results show that to prevent the entry and establishment of the bacterium in new areas and its potential spread to other crops, control of ‘Ca. L. solanacearum’ in seed lots is required.

Search for potential vectors of ‘Candidatus Liberibacter solanacearum’: population dynamics in host crops

“…Consequently, a lower prevalence of symptoms in celery was observed, whereas the presence of symptomatic carrots was similar to the previous years. This was likely, due to the primary infection caused by carrot seed transmission that was recently demonstrated (Bertolini et al, 2015). In both of the surveyed years, the arthropods collected on sticky celery plants showed the same psyllid species structure: B. trigonica was always the prevalent species and B. nigricornis was the less frequently observed species, with only two caught specimens in 2011.…”

“…For this reason, we focused our interest in psyllid species. Bertolini et al (2015) reported that the presence of B. trigonica and other species was correlated with an increase in the prevalence of the bacterium from 2% to approximately 100% after six months of carrot cultivation. Other experiences with emerging diseases in the citrus and potato industries suggest that psyllid species, feeding briefly outside their normal plant host range, could introduce a pathogen to another crop (Nelson et al, 2013).…”

“…The bacterium is primary transmitted by carrot seeds (Bertolini et al, 2015) and is afterwards transmitted by different psyllid species in a persistent way. Bactericera cockerelli Sulc is the vector of haplotypes A and B in solanaceous crops (Nelson et al, 2011).…”

Vector mediated transmission of Candidatus Liberibacter solanacearum and epidemiological consequences

“…In recent years, Lso has caused severe economic losses to the fresh carrot market in Norway, Finland, Germany and Spain. These losses can lead to important decrease in marketable yield (Munyaneza et al, 2010a;Laska, 2011;Teresani et al, 2014;Bertolini et al, 2015). Five haplotypes of Lso have been described so far.…”

“…Psyllids transmit several phloem restricted bacteria such as phytoplasmas and different species of the genus Candidatus Liberibacter (Bové and Garnier, 2003;Munyaneza, 2010;Trivedi et al, 2016). Among the Candidatus Liberibacter species, found in Europe, Candidatus Liberibacter solanacearum (Lso) is considered one of the most important ones due to the serious losses it cause to the carrot and celery industries (Munyaneza et al, 2010a;Bertolini et al, 2015). In Europe, two carrot psyllid species have been reported as vectors of Lso, Bactericera trigonica Hodkinson and Trioza apicalis Foester (Munyaneza et al, 2010a,b;Alfaro-Fernández et al, 2012b;).…”