Potential biocontrol microorganisms causing attenuated pathogenicity in Plasmopara viticola

A phenomenon of pathogenicity attenuation of was consistently observed during its subculture on grape. In order to clarify the causes of attenuated pathogenicity of , culturable microbes were isolated from the mass (mycelia, sporangiophores and sporangia, MSS) in each generation and tested for their biocontrol efficacies on grape downy mildew (GDM). The results showed that the incidence of GDM decreased with the increase in the number of subculture times on both vineyard collected leaves and grape leaves from in vitro grown seedlings. The number of culturable microbial taxa on the surface of decreased while the population densities of four specific strains, i.e., K2, K7, P1 and P5 increased significantly with the increase in subculture times. Compared with the control, biocontrol efficacies of the bacterial strain K2 reached 87.5% and those of both fungal strains P1 and P5 reached 100.0%. Based on morphological characteristics and molecular sequences, strains K2, P1 and P5 were identified as , and , respectively, and these three strains survived very well and multiplied on the surface of . As the number of times was subcultured increased, all three of these strains became the predominant strains, leading to greater inhibition, attenuated pathogenicity, and effective GDM biological control. To the best of our knowledge, this is the first report of and having biological control activity against GDM.