Carrot Pathogen ‘Candidatus Liberibacter solanacearum’ Haplotype C Detected in Symptomless Potato Plants in Finland

Abstract

Candidatus Liberibacter solanacearum' (CLso) haplotype C, a bacterial pathogen transmitted by the carrot psyllid Trioza apicalis, causes yield losses in carrot production. Due to concerns that this pathogen might also threaten potato (Solanum tuberosum) production, the occurrence of CLso in cultivated and volunteer potatoes in Tavastia Proper and Satakunta regions of Finland was studied. Volunteer potato plants were found in 13 of the 27 inspected carrot fields. Of the 148 potato samples tested by PCR, eight volunteer potato plants and one cultivated potato grown at the edge of a carrot field were found CLso positive. The PCR products obtained from these potatoes with primers OA2/OI2c, LpFrag4-1611F/LpFrag4-480R and CL514F/CL514R all showed 100% sequence identity to CLso haplotype C. This is the first observation of CLso haplotype C in field-grown potatoes. In addition, transmission experiments were performed. Attempts to transmit CLso into potato with carrot psyllids were not successful; however, CLso haplotype C was transmitted from infected carrots to potato plants by leaf grafting and by phloem connection formed by dodder, a parasitic plant, and found to survive in the potato plants for several weeks after transmission. However, the bacterial colonization progressed slowly in the potato phloem, and the amount of bacteria detected was low. The plants produced from the daughter tubers of the CLso positive potato plants were all CLso negative, suggesting that CLso haplotype C was not able to pass to the daughter plants. None of the CLso positive potatoes inoculated in greenhouse or collected from fields showed symptoms characteristic of zebra chip disease, associated with CLso haplotypes A and B.

‘Candidatus Liberibacter Solanacearum’ Is Unlikely to Be Transmitted Spontaneously from Infected Carrot Plants to Citrus Plants by Trioza Erytreae

“…In addition, previous studies using sampling methods at the level of the crop canopy have shown that the main psyllid species present in carrot fields is B. trigonica, followed by B. nigricornis in low proportions [27,29]. Several previous studies on bacterial transmission by psyllid species in nonhost plant species found that: T. apicalis (a vector of CaLsol in Apiaceae species in northern Europe) does not transmit the bacterium from carrot to potato [16], B. trigonica (a vector of CaLsol in Apiaceae species in southern Europe and North Africa) was able to transmit CaLsol at a low rate from carrot to potato or tomato [45,54], and the transmission of CaLsol to carrots by B. cockerelli (vector of CaLsol in Solanaceae species) was possible at a low rate and induced disease symptoms [62]. On the one hand, B. tremblayi could acquire CaLsol from infected carrot plants, but it could not transmit the bacterium to carrot plants, which might be because the bacterium was not able to complete circulation in the body of the psyllid to be ultimately inoculated from the salivary glands [45].…”

“…That is why, as authors, we believe that the CaLsol infection in citrus plants was not systemic. In a previous paper by Haapalainen et al [16], the transmission of CaLsol haplotype C through dodder and grafting onto potato was performed, but the transmission rate was very low; the bacteria did not colonize neither the root tissues nor the tubers. The reasons remain unknown, but they could be related to the specificity of the CaLsol haplotype, which could have different interactions with alternative hosts but also the cultivar assayed could be more resistant than others used in America [16].…”

“…Lane Late IVIA 188 on Citrange Carrizo). Plants growing in pots (Ø = 20 cm) and grown under controlled conditions (20 ± 5 • C, RH > 70%, 16:8 h (L:D) photoperiod). These citrus plants were regularly pruned to stimulate the emergence of new shoots.…”

“…Plant material of each assay was certified pesticide-free and was kept in pots under controlled conditions (20 ± 5 • C, RH > 70%, 16:8 h (L:D)) inside a mesh cage on insect-proof facilities. Plants were analyzed by real-time PCR according to the protocol designed by Teresani et al [10].…”

“…Trioza erytreae acquisition assays). All experiments were performed in a chamber under controlled conditions (20 ± 5 • C, RH > 70%, 16:8 h (L:D)). Calsol-free carrot plants without psyllid exposure were used in all the experiments as controls.…”