Geospatial mapping and biochemical characterization of walnut bacterial blight caused by Xanthomonas arboricola pv. juglandis in the temperate regions of Azad Jammu and Kashmir, Pakistan

Abstract

Bacterial blight of walnut (Juglans regia L.), caused by the Gram-negative bacterium Xanthomonas arboricola pv. juglandis, has recently emerged as a serious threat to walnut production in the temperate regions of Azad Jammu and Kashmir (AJK), Pakistan. The present study provides the first comprehensive surveillance and biochemical characterization of walnut bacterial blight across major walnut-producing districts of AJK. Systematic field surveys were conducted in eight districts, with ten locations surveyed per district, to assess disease prevalence, incidence, and severity on walnut leaves and fruits using a standardized 0-4 disease rating scale. The mean percent disease index (PDI) across the region ranged from 69.00% to 76.63%, indicating moderate to very high disease pressure. Disease prevalence varied between 60% and 100%, reflecting widespread distribution of the disease, while disease incidence reached 100% at several locations. The pathogen was consistently isolated from symptomatic tissues and produced characteristic yellow, mucoid colonies on nutrient agar and yeast dextrose calcium carbonate media. Biochemical characterization revealed typical reactions of Xanthomonas spp., including a Gram-negative reaction, positive catalase activity, and a negative oxidase test. Pathogenicity assays conducted on walnut leaflets and immature fruits successfully reproduced typical blight symptoms, and Koch’s postulates were fulfilled through re-isolation of the pathogen. Overall, the findings demonstrate severe disease pressure in the region and confirm X. arboricola pv. juglandis as the primary causal agent of walnut bacterial blight in AJK. This study provides a critical baseline for future epidemiological investigations and supports the development of integrated, region-specific disease management strategies.