Spread of Pseudomonas syringae pv. actinidiae in kiwifruit orchards: epidemiological role of the buds

Bacterial canker is a severe disease of kiwifruit caused by Pseudomonas syringae pv. actinidiae (Psa) which in Europe in 2013 was listed among the quarantine pests. In Italy the main factors which seriously damage kiwifruit industry are the fast spread, the strong pathogenicity and the problems in developing effective control strategies. To elucidate the role of the buds as penetration site of the pathogen within the disease life-cycle characterizing its epiphytic and endophytic phases, isolations from asymptomatic buds (≈600) and plant parts (shoot and sap) of Actinidia deliciosa cv. Hayward were carried out. In late winter 2014, 9 asymptomatic plants grown in a kiwifruit orchard that in the previous year had been slightly affected by bacterial canker in Emilia-Romagna region (northern Italy) were selected at given distances (<5m, within 15m and >20m) from a symptomatic plant (positive control). In 4 different survey times, every 15 days starting from the beginning of “bleeding”, from a one-year old shoot of each plant the sap was collected and afterwards each shoot was cut and analyzed. In all plant samples the Psa presence was determined by direct isolation on selective media and PCR analysis. At the end of the four surveys, the plants were monitored over the following months to highlight their phytosanitary state to relate the orchard situation with the effective absence/presence of Psa as pointed out by the early detection on “bleeding” and bud samples. In this preliminary study, the pathogen was present in buds from 3 out of 9 susceptible plants completely asymptomatic