Different symptoms in carrots caused by male and female carrot psyllid feeding and infection by ‘Candidatus Liberibacter solanacearum’

Abstract

Carrot psyllid Trioza apicalis was recently found to carry the plant pathogenic bacterium 'Candidatus Liberibacter solanacearum' (CLs). To confirm the transmission of bacteria by the psyllids and to dissect the symptoms caused in carrot plants by psyllid feeding and CLs infection, a greenhouse experiment with single psyllids feeding on separate plants was performed. A positive correlation was found between the amount of CLs bacteria in the psyllids and in the corresponding plants exposed to feeding, indicating CLs transmission. The female psyllid feeding caused more severe damage than male feeding, and resulted in a substantial decrease in the root weight. Female psyllid feeding also significantly reduced the carrot leaf weight and increased the number of curled leaves. The number of curled leaves was also increased by the nymphs when their number exceeded 10 per plant. A high titre of CLs bacteria significantly reduced root weight, while not affecting the weight or number of the leaves. However, the amount of CLs correlated with the number of leaves showing discolouration symptoms. Microscopy of infected carrot plants revealed that the phloem tubes throughout the whole plant, from leaf veins to the root tip, were colonized by bacteria. The bacterial cells appeared to be long and thin flexible rods with tapering ends and a transversally undulated surface. Microscopy also revealed collapsed phloem cells in the infected carrots. Damage in the phloem vessels is likely to reduce the sucrose transport from source leaves to the root, explaining the observed leaf discolouration and reduction in root weight.

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

“…During the seasonal arthropod surveys performed in celery crops in Villena in 2011 and 2012, 35.4% of the catches belonged to the superfamily Psylloidea although important differences were observed between both years in the number of specimens and the preva- (Bové, 2006;Cen et al, 2012), B. cockerelli in potato and tomato (Hansen et al, 2008) and T. apicalis in carrot (Nissinen et al, 2014). Moreover, recent studies suggest that Liberibacter species may be more widespread than previously thought, and vector species play an important role in bacterial spread.…”

Transmission of ‘Candidatus Liberibacter solanacearum’ by Bactericera trigonica Hodkinson to vegetable hosts

“…L. asiaticus' (Donovan et al, 2012), but the ability to transmit the bacterium was not reported. CaLso is transmitted by Bactericera cockerelli Sulc in potato and tomato (Secor et al, 2009) and by Trioza apicalis Förster in carrot in northern Europe (Nissinen et al, 2014). Bactericera trigonica Hodkinson is associated with vegetative disorders of carrot and celery in Spain (Alfaro-Fernández et al, 2012;Teresani et al, 2014), and Antolínez et al (2017aAntolínez et al ( , 2017b described the feeding behaviour of B. trigonica on carrot plants and reported that the transmission of CaLso was not affected by the vector gender.…”

“…CaLso is transmitted from potato mother tubers to growing plants and to progeny tubers (Pitman et al, 2011) and is a carrot seed borne pathogen , which poses a high risk of introduction of the bacterium into new areas in which contaminated carrot seeds are commercialized. The bacterium is also horizontally and naturally spread by different psyllid species (Hemiptera: Psylloidea) following establishment (Hansen et al, 2008;Alfaro-Fernández et al, 2012;Nissinen et al, 2014).…”

Vector mediated transmission of Candidatus Liberibacter solanacearum and epidemiological consequences

“…Two different psyllid species have been reported so far as vectors of Lso in carrots: Trioza apicalis Foerster in northern Europe (Munyaneza et al, 2010b;Nissinen et al, 2014;Munyaneza et al, 2015b) and Bactericera trigonica Hodkinson in the Mediterranean region (Alfaro-Fernández et al, 2012b). Due to the recent emergence of the bacterium, the mechanism by which carrot psyllids transmit Lso is not completely understood.…”

“…The reasons for such differences remain unexplained. However, it has been hypothesized that differences in the observed symptom severity could be the result of higher Lso titre inoculated due to an increased salivation time by females (Nissinen et al 2014 …”