Colonisation of sweet cherry buds (Prunus avium) by Pseudomonas syringae pathovars in commercial orchards in Central Otago

Bacterial canker of cherry, caused by Pseudomonas syringae pv. syringae (Pss) and P. s. pv. morsprunorum race 1 (Psm1), is a major constraint to growing sweet cherry in New Zealand. The importance of dormant buds in the disease cycle, as major overwintering sites for P. syringae pathovars, is well known. Less is known about the dynamics of both pathovars throughout the life span of the cherry bud. We observed the population dynamics of Pss and Psm1 on cherry buds in commercial orchards from initiation to budbreak (“the bud life cycle”). Cherry buds were sampled at regular intervals from their initiation (December) to budbreak (September), a total of 24 times during three growing seasons. Buds were macerated, total DNA extracted and presence of Pss and Psm1 determined using specific primers, and previously optimized qPCR. Cherry buds were colonised by Pss and Psm1 soon after formation. The bacterial population had established by bud scale lignification, which defined the overall bud infection for the season (i.e. there was no subsequent infection of the buds). The predominant pathovar on cherry buds was Pss. Both pathovars occurred commonly together in cherry buds in 2017, but not in 2018 and 2019. The knowledge generated in this study will inform the development of sustainable strategies, determine optimal timing for control, and help with new control tools for plant diseases caused by P. syringae