Heart Rot Disease of Walnut Caused by Nothophoma juglandis sp. nov. and Its Endophytic Biocontrol Agent

Abstract

English walnut (Juglans regia L.) is an economically important hardwood tree species cultivated worldwide. Walnut heart rot disease leading to heartwood decay of trees has been frequently observed in a number of plantations in China. To identify the causal agent, 29 diseased stem samples were collected from walnut plantations in Beijing, and 54 fungal isolates were obtained. Koch’s postulates were developed, and the results indicated that Nothophoma juglandis, a species new to science, was the causal agent of walnut heart rot disease. Granulobasidium vellereum, a notable biocontrol agent, was co-isolated with N. juglandis. Antagonistic assay on dual culture and walnut stems (both in field and detached branches) proved that G. vellereum acted as a potential biocontrol agent against N. juglandis, as it could significantly inhibiting the expansion of N. juglandis. The optimal temperature for mycelial growth and pathogenicity of N. juglandis were 26.6 °C and 27.0 °C, respectively, temperatures that frequently occur in the summer of the walnut growing regions in China.