Natural epidemic of fire blight in a newly planted orchard and effect of pruning on disease development.

After a severe fire blight outbreak in 2002 in Québec, growers underlined the need to develop a better understanding of the disease to both prevent and manage future outbreaks, with an emphasis on specific aspects on how and when to prune out diseased trees under our conditions. In 2005, a fire blight epidemic was triggered following delayed bloom in a newly planted orchard at the IRDA St-Bruno national park research station. This epidemic was an opportunity to compare pruning eradication strategies (Unpruned, Fast, Fast & Sterile, Late and Winter) under natural infection conditions. The experiment was setup as a completely randomized design with eight replicates, each including 10 trees. Disease development was closely assessed throughout the experiment. Tree mortality was highest for 'Late' treatment, in which trees were pruned starting one month after first disease detection. Disease incidence was lowest for 'Fast' and 'Fast & Sterile' treatments, in which trees were pruned weekly upon disease detection with shears sterilized or not. Pruning treatments had a marked effect on disease severity. The 'Unpruned' treatment had the highest severity whereas the lowest was recorded for the 'Fast' and 'Fast & Sterile' treatments. Overall, conditions observed in 2005 and 2007 were more conducive to disease than 2006. However, severity on affected trees was similar for every year, suggesting that weather has a lower impact on disease severity than on disease incidence. Overall, starting pruning immediately and then regularly after disease detection, was the best strategy to reduce disease intensity. There was no benefit to sterilize shears, so our future recommendations to grower will instead focus on the rapidity of interventions.