Cytospora pyri promotes Erwinia amylovora virulence by providing metabolites and hyphae1

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. amylovora and C. pyri. We first analyzed the coexistence frequencies of E. amylovora and C. pyri in 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. amylovora and C. pyri could coexist on the same lesion and caused much more severe disease. We also found that E. amylovora could physically attach to C. pyri and expression of amylovoran biosynthesis genes were up-regulated with fungal metabolite treatment. These results indicate that E. amylovora and C. pyri can cooperatively interact, which provides C. pyri with an opportunity to promote bacterial dispersal and production of virulence factor in E. amylovora.