Identification and evaluation of formulation additives to enhance foliar efficacy of a novel microbial biocontrol agent against Oomycetes.

Abstract

Conventional and organic farming systems are under pressure as European authorizations on pesticides are becoming increasingly restrictive, reducing the options available for controlling plant diseases. A new bacterial strain recently demonstrated efficacy in fungal disease control. However, the efficacy of microbial biocontrol agents observed under laboratory settings is often difficult to replicate in field conditions. This study aims to identify additives that could enhance the bacterium’s survival and proliferation under stress conditions on foliar tissues.

The first aspect of the work focuses on the selection of the additives, first through available literature and selection criteria, then through a toxicity test to ensure their compatibility with the bacterium. Then, the additives were tested to assess their ability to preserve the viability of the bacterium under abiotic stress conditions in vitro (UV, desiccation). In parallel, Biolog tests were carried out to determine carbon and nitrogen sources that could directly enhance the bacterium's proliferation. Finally, whole-plant assays were conducted on the pathosystem Solanum lycopersicum – Phytophthora infestans to confirm the effect of the additives in improving bacterial survival and its biocontrol activity.

The study revealed that sodium-lignosulfonate (LS) successfully protects the bacterium from mortality caused by UV radiation, as 17.6% of the initial population was maintained after 60 minutes exposure, compared to a survival of less than 5 minutes (0%) for the control population. Similarly, polyethylene glycol 6000 (PEG) appears to protect the strain from desiccation, with respective survival rates of 96.4% and 17.8% after 15 and 45 minutes of exposure, in contrast to a survival rate of 6.59% after 15 minutes for the control population. These additives, when evaluated on the Solanum lycopersicum model, exhibited no phytotoxicity and even appeared to protect the plant from necrosis symptoms observed with the control (isotonic water) treatment. However, they failed to protect the bacterial strain under strong desiccation stress in the growth chamber, as survival rates on the foliar surfaces remained below the detection threshold in both assays. Furthermore, the plant-pathogen assay revealed that the supplemented strain was unsuccessful in reducing disease symptoms compared to the control (isotonic water).

In conclusion, these results enabled the identification of promising additives for protecting the novel biocontrol agent against UV radiation and desiccation stress. However, the tested additive combination did not allow satisfying results on the in vivo model. This work initiates the development process of this biocontrol agent and provides promising perspectives. The results highlight the relevance of further in vivo investigation of PEGs with different molecular weights, as well as additional research on carbon and nitrogen sources, with the aim of finding the best additive combination for a robust final product.