Walnut blight, caused by Xanthomonas arboricola pv. juglandis (Xaj) is the most destructive bacterial disease of walnut fruit worldwide. In years of high disease pressure, 100% incidence of fruit and greater than 80% of crop losses have occurred in Australia. Current chemical management strategies for walnut blight in Australia were developed in a cool-temperate region in Tasmania and primarily relies on preventative spray applications of copper-mancozeb mixtures, applied from budburst (stage Cf2) and thereafter at 7–14 day intervals and/or before rainfall. The effectiveness of this spray programme, however, has not been evaluated in the semi-arid climate of Australia, a region where greater than 90% of walnuts are grown in Australia. Hence, this study examined the effectiveness of copper-mancozeb in a semi-arid region (Leeton, NSW), covering four phenological stages (budburst, flowering, fruiting and shell hardening) of walnut cultivar Lara during the 2024-25 season. The trial was undertaken in a commercial walnut orchard, with randomized complete block design including five treatments and five replicates. Treatments were applied to three-tree plots, adjacent to each other within a tree-row. Disease incidence was assessed on 120-150 fruits per plot on the middle tree only from the onset of fruiting (stage Gf) until crop maturity. The results showed that the treatments that commenced from the onset of budburst (Cf2), flowering (Ef) and fruiting (Gf) to shell hardening (SH) stage significantly reduced the mean disease incidence from 71.4% in the untreated control to 39.6%, 46.7% and 49.1%, respectively. In contrast, disease incidence (72.8%) was similar to the control where treatments were only applied from budburst (Cf2) to the onset of flowering (Ef). The rate of disease progress, calculated as the percentage of standardised area under the disease progress curve (SAUDPC%) was lowest in trees sprayed from Cf2 (33.8%), Ef (40.5%) and Gf (43.3%) to SH stage. In addition, copper-mancozeb applied from Cf2 to SH showed significant (p<0.05) effect in supressing the growth of diseased-induced premature fruit drop. These findings provide insight into the critical infection period of Xaj and susceptible stages of walnut under field environment in commercial walnut orchards. Studies are ongoing to determine source of inoculum, transmission pathway and environmental factors associated with walnut blight infection and disease development in semi-arid region.