Azospirillum brasilense Sp7 and tomato seedling exudate or cytidine reduced disease symptoms on yellow pear tomatoes caused by Pseudomonas syringae pv. tomato DC3000

Tomatoes are an important crop worldwide and phytopathogens can cause devastating losses. Our study describes a combination treatment for tomatoes, consisting of pro-biotic Azospirillum brasilense Sp7 and either complete tomato seedling exudate or the single exudate compound cytidine. The combination of A. brasilense Sp7 with cytidine showed a remarkable reduction of 83.4% in disease symptoms of the tomatoes that had been challenged with Pseudomonas syringae pv. tomato DC3000. Replacing cytidine with tomato seedling exudate was slightly less effective at 71%, but the reduction in disease symptoms was still larger than with A. brasilense Sp7 alone at 55%. This reduction in disease symptoms was not paralleled by a reduction in Pseudomonas live counts, abundance of P. syringae pv. tomato DC3000 16S rDNA, or copies of the P. syringae hrpL gene in leaf homogenates. The symptom reduction was paralleled by a 6.7 fold increase in A. brasilense Sp7 16S rDNA in root homogenates in response to germinating the seeds in cytidine. In the phyllosphere, bacterial 16S rDNA could only be detected in leaf homogenates from plants that were challenged with P. syringe pv. tomato DC3000. In phyllosphere and rhizosphere, treatments modulated the microbiome. At the species level, A. brasilense together with cytidine reduced the presence of Stenotrophomonas maltophilia, a human pathogen, in the phyllosphere. In the rhizosphere, adding A. brasilense Sp7 might have caused a switch from Rhizobiales to Azospirillales.