Widespread occurrence of 'Candidatus Phytoplasma ulmi' in elm species in Germany

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. Results: About 6,500 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.

Genetic variation, phylogenetic relationship and spatial distribution of ‘Candidatus Phytoplasma ulmi’ strains in Germany

“…It is conceivable that an even greater number of genotypes would have been identified, as the examined accessions represented only 16% of the ‘ Ca. P. ulmi’-positive samples identified in the previous survey 15 . However, samples were carefully selected based on an even geographical spread and on a representative number of samples from all three elm species.…”

“…Identification of ‘ Ca. P. ulmi’ was performed by a pathogen-specific quantitative real-time PCR assay described by Schneider et al 15 . DNA from the 16SrV-C group phytoplasma alder yellows, strain ALY 16 and the 16SrV-A group phytoplasma ‘ Ca.…”

“…It took more than 20 years until a local survey demonstrated the presence of ‘ Ca. P. ulmi’ in European white elm trees in East Germany 13 , and the fact that all phytoplasma-positive trees lacked typical disease symptoms prompted a nationwide survey in native elm habitats and revealed an average infection rate of 28% 15 . This study also corroborated the observation of the previous survey that symptoms are rare, a circumstance explaining why the infections went unnoticed for such a long time.…”

Multigene characterization of 'Candidatus Phytoplasma ulmi'‐related isolates associated with elm yellows disease of Ulmus minor Mill. in Poland

“…laevis and U . glabra were tested positively (Schneider et al, 2020). Less samples were collected in Croatia; nevertheless, the high prevalence of ‘ Ca .…”