Abstract:

Fire blight and pear Valsa canker，two prominent diseases，currently inflict substantial damage on the production of Korla fragrant pear（Pyrus sinkiangensis）. Their often co-occurrence in the field，exacerbating the severity of the diseases and complicating control efforts. In order to clarify the interaction between the fire blight pathogen bacterium（Erwinia amylovora，E.a）and the pear Valsa canker pathogen fungus（Valsa pyri，V.p），and to explore the complex infection mechanisms of these pathogens，the present study was conducted to investigate the effects of pathogen interactions on the growth and pathogenicity of E.a and V.p by using E.a strain E.a001 and V.p strain V.pKRL fermentation filtrates co-culture. The results showed that co-culture of E.a001 with V.pKRL fermentation filtrates significantly enhanced the growth rate of E.a001，with a higher viable cell count compared to the control，and a 2.2-fold increase in biofilm formation compared to the control. Co-culture of V.pKRL with E.a001 frmentation filtrates resulted in a 1.46-fold increase in V.pKRL mycelial biomass，and a 14.9% increase in conidial germination rate compared to the control. Additionally，both co-culture of E.a001 with V.pKRL and V.pKRL with E.a001 fermentation filtrates reduced the pH of the culture medium，whereas co-culture of E.a001 with the reference strains of Alternaria alternata，and V.pKRL with Pseudomonas syringae and Dyckeya fangzhongda sp. nov did not exhibit growth promotion or pH reduction characteristics. It was found that co-culture of E.a001 + V.pKRL fermentation filtrate increased the extracellular polysaccharide and pear fire blight toxin production of E.a001. The relative expression of HrpA，hrpB，hrpD，hrpJ，HrpW，hrpY and hrcJ in the hrp gene cluster was up-regulated by 1.53 to 2.27-fold，with the greatest up-regulation of the hrpY gene. Similarly，E.a001 fermentation filtrate increased the synthesis of V.pKRL phlorizin degradation toxins，and the relative expressions of cell wall degradation-related enzyme genes，PG，β-Gal，PME and α-Af were up-regulated by 1.64 to 1.91-fold，with the largest up-regulation of the PG gene. Pathogenicity assessments on Korla fragrant pear ranches，leaves，and fruits revealed that sequential composite inoculation of E.a001 and V.pKRL at the same site induces the most potent pathogenicity，marked by rapid symptom development，lesion expansion，and increased V.p spore production compared to single or sequential inoculations at different sites. These findings underscore mutual promotion between E.a and V.p，enhancing toxin production，up-regulating pathogenic gene expression，and reinforcing pathogenicity，thereby indicating a specific cooperative interactions relationship between the two pathogens.

Key words: pear, Erwinia amylovora, Valsa pyri, co-infection, bacterial-fungal interactions, pathogenicity