Cytospora pyripromotes Erwinia amylovoravirulence by providing metabolites and hyphae

Bacterial–fungal interactions are widespread in nature. We observed that pear orchards affected by Cytospora pyri(formerly Valsa pyri) were often accompanied with Erwinia amylovora. However, the relationship of the two pathogens was unclear. The objective of this study was to determine whether the synergistic effect exists between E. amylovoraand C. pyri. We first analyzed the coexistence frequencies of E. amylovoraand C. pyriin pear trees. Virulence of the two pathogens, growth, physical interactions, amylovoran production, and expression of genes for amylovoran biosynthesis were conducted. Our results showed that E. amylovoraand C. pyricould coexist on the same lesion and caused much more severe disease. We also found that E. amylovoracould physically attach to C. pyriand the expression of amylovoran biosynthesis genes were up-regulated with fungal metabolite treatment. These results indicate that E. amylovoraand C. pyrican cooperatively interact, which provides C. pyriwith an opportunity to promote bacterial dispersal and production of virulence factor in E. amylovora.