Use of a growth regulator (prohexadione-Ca) and summer pruning as post symptom rescue treatments following a fire blight infection during bloom

Fire blight of apples (Erwinia amylovora) can be a serious disease depending on production area and year. Bloom is the most frequent starting point for infection and the primary focus for sprays which can prevent infection, or of growth regulators such as prohexadione-Ca (ProCa) to limit spread. Because of the sporadic cases, many growers opt to forego bloom sprays when disease pressure is low. When symptoms do appear, quickly pruning out affected branches is currently the most effective control measure. Our goal was to determine if ProCa applied when symptoms appear, instead of during bloom when infection risks are predicted, could be used to limit disease spread, and if this would interact with summer symptom removal. The study was carried out in 2018 and 2019 at the IRDA orchard in Saint-Bruno-de-Montarville, Quebec. Plots inoculated during bloom were allocated to treatments in which diseased branches were either pruned during summer or only in winter, and trees sprayed with ProCa starting at bloom, when symptoms appeared, or not. Potential for disease progression was assessed by inoculating shoots and rating them for disease severity and ooze production. Significantly less wood was removed from trees pruned during the summer as opposed to winter, but total yield was reduced. ProCa had no effect on the amount of dead wood pruned, but still reduced disease severity. We found that ProCa reduced ooze on inoculated shoots and that post symptom timing was as effective as the bloom spray. Delaying sprays until symptoms appear could become an alternative strategy to reduce summer spread of fire blight.