Action of Pythium oligandrum on Grapevine Trunk Diseases and its impact on microbial communities

Grapevine Trunk Diseases (GTDs) have become a major challenge for viticulture. Since the ban of sodium arsenate (the only approved pesticide) in 2001, the need for alternative methods, such as biocontrol, has become increasingly urgent. One promising microorganism is the oomycete Pythium oligandrum, which is known to improve plant health by increasing natural defences and reducing disease incidence by up to 40 %. In order to use this microorganism in vineyards, it need first to be formulated and tested to ensure its safety. An experiment was therefore carried out in a greenhouse under semi-controlled conditions to assess the efficacy of the P. oligandrum biopesticide produced and its impact on microbial communities using a high-throughput sequencing approach. Vines were treated with the biopesticide and were inoculated with two fungi involved in wood diseases: Neofusicoccum parvum (involved in Botryosphaeria dieback) and Phaeomoniella chlamydospora (involved in Esca). During a three-month experiment, samples of the rhizosphere environment were collected to assess possible changes in microbial communities, either as part of GTDs or as a result of the action of P. oligandrum. The results indicated a minimal effect of the P. oligandrum biopesticide on the total microbial community of the vine rhizosphere. However, the treatment appeared to enhance several beneficial taxa that play a role as plant growth promoting rhizobacteria (PGPR) or biocontrol agents (BCA). This improvement, together with the direct effect of P. oligandrum, may explain the reduction in necroses caused by N. parvum and P. chlamydospora following the biopesticide application.