Plant colonization and survival of Pseudomonas synxantha DLS65, the active ingredient of the biopesticide Microfighter: essential keys for optimized and effective field applications.

Copper-based pesticides are widely used to control bacterial and fungal diseases of key crops in Europe, such as downy mildew on grape, bacterial speck on tomato and olive knot disease. Widespread use of copper, however, is negatively affecting the soil microbiota in terms of abundance and biodiversity; therefore, it is not environmentally sustainable. As an alternative to control such diseases, the EU-funded project “LIFE Microfighter” has led to develop an innovative biopesticide named ‘Microfighter’, in which Pseudomonas synxantha strain DLS65 is the microbial active ingredient. In the current study, we assessed the colonization pattern and survival of DLS65, on tomato, olive, and grapevine. Under controlled conditions, DLS65 successfully colonized both the epiphytic and endophytic compartments of tomato leaves (cv. Roma), after one week post inoculation, with an abundance of 2.2 × 103and 4.3 × 104CFU g-1leaf fresh weight, respectively. However, in the root compartment, DLS65 was only observed in the rhizosphere, with an abundance of 5.9 × 106CFU g-1root fresh weight. Microscopic visualization of eGFP-tagged DLS65 with a confocal laser scanning microscopy confirmed both rhizospheric and phyllospheric competence of Pseudomonas synxantha in tomato. In field condition, after a foliar application of ‘Microfighter’, strain DLS65 exhibited crop-dependent survival within the phyllosphere of olive and grapevine plants. Competent colonization and bacterial survival on target crop plants are essential prerequisites towards an efficient application of the biopesticide in field: for instance, the establishment of the most suitable/effective concentration per gram of strain DLS65 viable cells in the product and the most ppropriate product spray intervals. Research funded by the European Commission under the LIFE21-ENV-IT-LIFE MICROFIGHTER project, Grant Agreement N. 1,010,742,018. Research was conducted within the framework of the EU-COST action CA22158 (MiCropBiomes: Exploiting Plant-Microbiomes Networks and Synthetic Communities to improve Crops Fitness).