What is the risk of pathogens for trees and forests in Northern Europe? Case studies of three pathogens: Brenneria salicis, Xylella fastidiosa, and Ceratobasidium theobromae

Details

Supervisors

Bragard, Claude ; Vinatzer, Boris

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en sciences agronomiques, à finalité spécialisée

Abstract

Forests are very important ecosystems present all over the world. They provide many human services such as supporting agricultural and industrial needs. Emergent pathogens causing new diseases in forest ecosystems have been increasingly detected over the past century. In this context, three forestry pathogens have been selected for this master’s thesis: Xylella fastidiosa, Brenneria salicis, and Ceratobasidium theobromae. X. fastidiosa is a bacterium initially discovered in California (USA) in the late 19th century. It was later first identified in Italy in olive tree, causing olive quick decline syndrome. This bacterium is now regarded as one of the major phytosanitary threats in Europe. Brenneria salicis is a bacterium first reported in the early 20th century, causing Watermark disease, the most serious bacterial disease affecting willows. Finally, Vascular Streak Dieback (VSD), a new disease named after its origins in South Asia, was discovered in Virginia and other states in the US. Although the causal agent has not been definitively identified and Koch’s postulates have not yet been established, Ceratobasidium theobromae is consistently detected in VSDinfected trees and is considered a major potential pathogen. The objective of this master’s thesis is to assess the interactions between pathogens and microbial communities in relation to disease expression. B. salicis serves as a prediction model for X. fastidiosa future establishment in Belgium. Indeed, this bacterium infects trees from Salix and Populus genera, identified as important potential hosts for X. fastidiosa in Belgium. In addition, both bacteria share similarities, such as similar colonization pathways and symptomless behavior on various infected trees. Both bacteria were inoculated into tobacco plants along with pre-selected endophytes. Pathogen colonization within the vessels was monitored using PCR to gain insights into disease expression. The results indicated that the two bacteria colonized the plants at different rates, with Bacillus amyloliquefaciens emerging as a significant factor, potentially delaying the onset of infection. This bacterium is noted in the literature for its promising roles in plant growth promotion and biocontrol in agriculture. To gain more information about VSD, the bacterial communities of diseased trees were studied. Following the sequencing of VSD-infected samples, the bacterial communities were classified using bioinformatics tools such as Kraken2, Minimap2, Sourmash, and Flye. Two bacteria were confidently assembled into genomes: Achromobacter insuavis and an unnamed species, Stenotrophomonas sp002192255. Both were found in high abundance in some of the cultivated mycelium samples but were not directly present in the plant samples. Their presence in the mycelium might be due to symbiotic or commensal interactions with Ceratobasidium theobromae. Additionally, Stenotrophomonas maltophilia, the closest NCBI organism to Stenotrophomonas sp002192255, supports the growth of Candidatus Liberibacter asiaticus and led to partially fulfilled Koch’s postulate. The presence of a closely related Stenotrophomonas species could provide a potential lead toward completing Koch’s postulates for C. theobromae in relation to VSD.