Pathogenic Oomycota species in walnut nurseries of Bursa and Isparta(Eğirdir) provinces in Türkiye, with a first report of Phytopythium vexans, Phytopythium litorale, Pythium aphanidermatum and Globisporangium ultimum on walnut

Türkiye holds a significant position in global walnut production. According to data from the 2023 production year, the country ranks sixth worldwide, producing approximately 67.000 metric tons of walnuts. However, the expansion of walnut cultivation has also introduced new challenges, particularly root rot diseases that threaten the health and productivity of walnut orchards. One of the most critical diseases affecting walnut crops globally is walnut dieback. This study aimed to characterize the symptoms of walnut dieback and identify the associated causal agents in walnut nurseries located in the Bursa and Isparta (Eğirdir) provinces of Türkiye. Field surveys were conducted between 2019 and 2022 in nearly all commercial walnut nurseries in Bursa, as well as in a major nursery in Isparta (Eğirdir). Plants exhibiting symptoms such as general growth suppression, dieback, leaf yellowing and discoloration, reduced leaf count, and desiccation were examined, focusing on their roots and root collars. In vivo assays and molecular analyses identified several oomycete species, including Phytophthora, Phytopythium, Pythium, and Globisporangium, as the causal agents of the disease. The coexistence of multiple pathogenic oomycetes raises concerns regarding orchard health and the potential transmission of these pathogens to other fruit crops in the region. These findings will assist growers in adopting improved cultural practices and implementing effective management strategies to ensure the long-term sustainability of walnut production in Türkiye.