Fire blight, caused by the bacterium Erwinia amylovora , is an important disease affecting many rosaceous plants, including apple and pear, that can lead to devastating economic losses worldwide. Similar to many xylem-invading pathogens, E. amylovora forms biofilms that rely on the production of exopolysaccharides (EPSs).

Resistance to sterol demethylation inhibitor fungicides (DMIs) in Monilinia fructicola, causal agent of brown rot of stone fruit, has been reported in the southeastern and eastern United States and in Brazil. DMI resistance of some M. fructicola isolates, in particular those recovered from the southeastern U.S., is associated with a sequence element termed ‘Mona’ that causes overexpression of the cytochrome demethylase target gene MfCYP51. In this study, we conducted statewide surveys of Michigan stone fruit orchards from 2009-2011 and in 2019, and determined the sensitivity to propiconazole of a total of 813 isolates of M. fructicola. A total of 80.7% of Michigan isolates were characterized as resistant to propiconazole by relative growth assays but the ‘Mona’ insert was not uniformly detected, and was present in some isolates that were not characterized as DMI resistant. Gene expression assays indicated that elevated expression of MfCYP51 was only weakly correlated with DMI-resistance in M. fructicola isolates from Michigan, and there was no obvious correlation between the presence of the ‘Mona’ element and elevated expression of MfCYP51. However, sequence analysis of MfCYP51 from 25 DMI-resistant isolates did not reveal any point mutations that could be correlated with resistance. Amplification and sequencing upstream of MfCYP51 resulted in detection of DNA insertions in a wide range of isolates typed by DMI phenotype and the presence of ‘Mona’ or other unique sequences. The function of these unique sequences or their presence upstream of MfCYP51 cannot be correlated to a DMI-resistant genotype at this time. Our results indicate that DMI resistance was established in Michigan populations of M. fructicola by 2009 to 2011, and that relative resistance levels have continued to increase to the point that practical resistance is present in most orchards. In addition, the presence of the ‘Mona’ insert is not a marker for identifying DMI-resistant isolates of M. fructicola in Michigan.

Summary Erwinia amylovora, the causative agent of fire blight, uses flagella‐based motilities to translocate to host plant natural openings; however, little is known about how this bacterium migrates systemically in the apoplast. Here, we reveal a novel surface motility mechanism, defined as sliding, in E. amylovora. Deletion of flagella assembly genes did not affect this movement, whereas deletion of biosynthesis genes for the exopolysaccharides (EPSs) amylovoran and levan resulted in non‐sliding phenotypes. Since EPS production generates osmotic pressure that potentially powers sliding, we validated this mechanism by demonstrating that water potential positively contributes to sliding. In addition, no sliding was observed when the water potential of the surface was lower than −0.5 MPa. Sliding is a passive motility mechanism. We further show that the force of gravity plays a critical role in directing E. amylovora sliding on unconfined surfaces but has a negligible effect when cells are sliding in confined microcapillaries, in which EPS‐dependent osmotic pressure acts as the main force. Although amylovoran and levan are both required for sliding, we demonstrate that they exhibit different roles in bacterial communities. In summary, our study provides fundamental knowledge for a better understanding of mechanisms that drive bacterial sliding motility.