Widespread occurrence of \u2018Candidatus Phytoplasma ulmi\u2019 in elm species in Germany

Bernd Schneider,Michael Kube,Ralf Kätzel

doi:10.1186/s12866-020-01749-z

Bernd Schneider, Michael Kube + Show 1 more

Open Access

https://doi.org/10.1186/s12866-020-01749-z

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Abstract

Background‘Candidatus Phytoplasma ulmi’ is the agent associated with elm yellows and has been categorised in the European Union as a quarantine pathogen. For central and northern European countries, information on the occurrence and distribution of the pathogen and its impact on elms is scarce, so a survey of native elm trees has been conducted in Germany.ResultsAbout 6500 samples from Ulmus minor, Ulmus laevis and Ulmus glabra, were collected nationwide. Phytoplasma detection was performed by applying a universal 16Sr DNA-based quantitative PCR (qPCR) assay and a novel ‘Ca. P. ulmi’ specific qPCR assay targeting the 16S–23S spacer region. Both assays revealed that 28% of the samples were infected by ‘Ca. P. ulmi’, but infection rates of the elm species and regional incidences differed. The phytoplasma presence in the trees was not correlated to disease-specific symptoms. The survey identified a regional disparity of infection which was high in east, south and central Germany, whereas only a few infected sites were found in the western and northern parts of the country. Monitoring the seasonal titre of ‘Ca. P. ulmi’ in an infected tree by qPCR revealed a high colonisation in all parts of the tree throughout the year.Conclusions‘Ca. P. ulmi’ is widely present in elms in Germany. The rare occurrence of symptoms indicates either a high degree of tolerance in elm populations or a low virulence of pathogen strains enabling high infection rates in a long-living host.

Highlights

Phytoplasmas are obligate parasites from the bacterial class Mollicutes, where they form the monophylogenetic taxon ‘Candidatus Phytoplasma’ [1]
Elm species were infected to a different degree with regional disparity of infection
Despite the high infection rate, disease symptoms were rarely found indicating a high degree of tolerance of native elm species to infection

Summary

Introduction

Phytoplasmas are obligate parasites from the bacterial class Mollicutes, where they form the monophylogenetic taxon ‘Candidatus Phytoplasma’ [1]. They colonise the nutrient-rich phloem sap of their plant host and rely for transmission on phloem-feeding hemipteran insect vectors [2]. Most North American elm species are highly susceptible to an infection by the bacterium and show a dramatic course of disease progression [12,13,14]. The trees usually die within two years post-infection, displaying a number of characteristic symptoms such as leaf yellowing, witches’ broom formation and phloem necrosis. The disease symptoms displayed by the European elm species resembled those of the North American elm species, but phloem necrosis did not occur. The European elm species were considered less susceptible than their American relatives [18]

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