Distribution of Xanthomonas arboricola pv. juglandis and Alternaria species-groups populations associated with brown apical necrosis at different phenological stages of walnut trees

Abstract

Walnut (Juglans regia) can be affected by several diseases. Among them, walnut blight (WB) which is caused by the bacterium Xanthomonas arboricola pv. juglandis (Xaj) and brown apical necrosis (BAN) which has lately become economically significant causing crop losses in recent years. BAN can be considered as a complex disease in which, besides Xaj, the fungal genera Fusarium and Alternaria could be also involved. The objective of the present work was to determine the occurrence of Xaj and Alternaria sp.-grps. at different phenological stages of walnut in order to provide knowledge about the epidemiological aspects of BAN. Xaj was detected at 9 of the 11 phenological stages under study and Alternaria at all stages. No Fusarium isolates were detected at any stage. Buds were the main hibernation site for the bacterium which showed higher epiphytic concentrations than isolates of Alternaria. However, the fungus was more frequently found at all phenological stages until fruit set. On diseased fruits, the bacterium was predominant and the fungus was only found sporadically. This work provides knowledge that can contribute to the improvement of preventive programs such as control strategies that consider the weather forecasts and the phenology of walnut instead of strategies based on fix calendars.