HEAT-TREATMENT OF PLANT-PROPAGATION MATERIAL FOR THE CONTROL OF FIRE BLIGHT

The thermal sensitivity of Erwinia anzylovoia was determined at 45 degrees C and 50 degrees C. In vitro assays with eight different strains showed that thermal death times did not exceed 70 min at 45 degrees C and 50 min at 50 degrees 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 degrees C. When shoots were sealed in polyethylene bags and immersed in water (dry heat), no Erwinia amylovora could be isolated after an immersion of 3 h at 45 degrees C. Incubation at 50 degrees C for 1-2 h 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.