Virulence and Pathogenicity of Xanthomonas arboricola pv. juglandis Causing Walnut Blight Disease

Abstract

Walnut blight, incited by Xanthomonas arboricola pv. juglandis (Pierce) Vauterin et al., constitutes one of the most economically consequential bacterial diseases affecting commercial walnut (Juglans regia L.) orchards, causing severe losses through infection of expanding shoots, leaves, catkins, and developing nuts under conditions of high humidity and moderate temperatures during the growing season. Despite the widespread distribution of this pathogen across major walnut-producing regions, comprehensive assessment of virulence variability and pathogenicity differences among naturally occurring pathogen populations remains critically important for resistance breeding programs and the rational design of disease management strategies. This study evaluated the virulence and pathogenicity of X. arboricola pv. juglandis isolates recovered from symptomatic walnut tissues exhibiting characteristic blight lesions across geographically distinct orchard sites. Bacterial isolates were subjected to cultural, biochemical, and physiological characterization prior to pathogenicity confirmation through artificial inoculation of detached walnut leaves, green husks, and intact shoots under controlled greenhouse conditions following Koch's postulates. Virulence differentiation among isolates was quantitatively assessed based on lesion expansion rates, disease severity indices, and latent period measurements recorded at standardized inoculation densities. Molecular characterization of representative isolates was performed using repetitive element palindromic PCR fingerprinting and multilocus sequence analysis to elucidate genetic diversity and population structure in relation to virulence phenotype. Significant intrapathovar variation in aggressiveness was detected among isolates, indicating the existence of distinct virulence groups with differential host colonization capacities. These findings provide essential pathological and epidemiological insights supporting the development of resistant walnut cultivars and evidence-based bactericide application programs for walnut blight management.