Abstract
The walnut (Juglans regia L.) is an economically important crop in southern Kazakhstan, where brown spot disease, caused by the fungus Alternaria alternata, poses a serious threat to yield. The objective of this study was to determine whether A. alternata isolates obtained from walnut leaves in orchards located in different climatic zones (Almaty and Turkestan regions) exhibit pathogenic properties in all cases. The fungus was identified based on morphological characteristics and confirmed by PCR analysis. Pathogenicity was assessed through artificial inoculation of walnut leaves under controlled conditions. The results demonstrated that A. alternata exhibits pathogenicity under diverse climatic conditions, inducing brown spot symptoms, including chlorosis and necrosis of leaves. These findings confirm the wide distribution and threat of brown spot and provide a scientific basis for further research aimed at developing effective plant protection strategies.