Risk of Erwinia amylovora transmission in viable but nonculturable (VBNC) state via contaminated pruning shears

Various phytopathogenic bacteria, including Erwinia amylovora which causes fire blight in species belonging to the Rosaceae family, adopt a viable but nonculturable (VBNC) state that survives but cannot grow in nutrient media when the external environment is not suitable for survival. The fire blight pathogen enters the VBNC state when exposed to various stresses, such as copper shock, starvation, and oxidative stress. The cultivation of apple and pear orchards by pruning twigs and blooms in Korea is often carried out by workers who move through several adjacent orchards. This increases the risk of transmission of fire blight pathogen on pruning shears during hand-pruning and thinning operations. In particular, the risk of spreading the fire blight pathogen in its VBNC state is increased due to starvation and dry stress on the pruning shears when a healthy tree in an orchard is pruned after pruning of symptomatic trees. Under laboratory conditions, after spraying the fire blight pathogen on pruning shears, E. amylovora lost culturability after 24 h, and a VBNC state was confirmed physiologically. Culturability was subsequently restored through inoculation of liquid medium and host plants, and so a VBNC state for E. amylovora on pruning shears was finally confirmed. Thus, our findings suggest that it is essential to sterilize pruning shears and farm equipment used by orchard workers to minimize the risk of spreading E. amylovora.