Abstract:

Objective: The aim of this study was to understand the prevelance and incidence of Juglans leaf necrosis (JLN) in Xinjiang, investigate the disease dynamics and its correlation with phyllosphere microbiome and mineral elements, and determine the key precipitating factors of JLN in walnut in Xinjiang, which could provide information for its monitoring, forecasting, and prevention. Method: We selected 291 walnut orchards from 17 counties in the main planting areas of walnut in South of Xinjiang. Conducted on-site investigations on the current situationof JLN. The disease rating scale of the leaf necrosis was established based on severity. The structure and composition of microbial communities inhabiting the phyllosphere, root endosphere and rhizosphere soil of healthy walnuts and the ones suffered from leaf necrosis were analyzed, and the pathogenicity of walnut-associated fungi was performed. Linear regression analysis was used to reveal the correlation between the content of mineral elements in leaves and the severity of leaf necrosis. Result: The results showed that the JLN occurred approximately in the stop-growing period of the new shoots and reached the peak before the germination of the secondary shoots, where the leaf margins or tips initially appeared withered and yellow (or necrosis). This symptom then gradually spread towards the leaf core. Leaves with mild necrosis showed, the symptom manifested as the scorch of leaf edge or tip, while on the leaves of severe necrosis, the whole leaf was scorched and the plant was damaged heavily. No wilted leaves can be observed before occurrence of leaf necrosis, while fallen leaves were rarely detected after the symptom showed up. Juglans leaf necrosis had no obvious disease center or infectivity. The symptom can be rated on a five-level scale based on the incidence and severity. The results of microbiome analysis showed that Halomonas, Pseudomonas and Haliangium were significantly enriched that could tolerate high salt stress in the phyllosphere, root and rhizosphere soils with pyrophyllosis. Leaf fungi, including Alternaria, Colletotrichum and those belonging to Didymellaceae were significantly enriched in the phyllosphere. The results of leaf pathogenicity detection showed that these three types of leaf fungi were not the key biological factors causing leaf necrosis. The contents of O₂^? and H₂O₂ in the leaves of walnut suffered from leaf necrosis were significantly increased, furthermore, the quantities of Cl^?, Na and B elements in the diseased leaves rose significantly with the increasement of severity of leaf necrosis, but the contents of N, K and Fe elements in these leaves were remarkably decreased. Conclusion: Bacterial communities tolerant to high salt stress were significantly enriched in the phyllosphere, root and rhizosphere soil where leaf necrosis occurred. Pathogenic fungi were not the main biological factors causing scorchleaf disease. We speculate that toxicity of Cl^?, Na, B and insufficient of N, K, Fe in leaves might be the key cause of the Juglans leaf necrosis. Our results demonstrated that Juglans leaf necrosis may be a physiological disease caused by abiotic factors.

Key words: Juglans regia, Juglans leaf necrosis, ranking standards, disease dynamics, physiological disease