Heat treatment of plant propagation material for the control of fire blight

The thermal sensitivity of Erwinia amylovora was determined at 45° C and 50°C. In vitro assays with eight differetit strains showed that thermal death times did not exceed 70min at 45° C and 50min at 50° C (95% confidence interval). Heat treatments of naturally infected apple and pear shoots were performed using a moist and a dry heat procedure. When shoots were wrapped in wet cotton cloths (wet heat) and maintained in an incubator, no bacterial growth could be detected after an incubation of 5 h at 45° C. When shoots were sealed in polyethylene bags and immersed in water (dry heat), no Erwinia amytovora could be isolated after an immersion of 3 h at 45° C. Incubation at 50°C for 1 –2h in either condition did not eradicate the pathogen entirely. The failure rate of grafts using budwoods treated at the effective time‐temperature combinations did not exceed 25%. The present investigations showed that the use of thermotherapy techniques for controlling Erwinia amylovora in propagation material of apple and pear might be possible.