Lasiodiplodia theobromae and Neoscytalidium dimidiatum associated with grafted walnut (Juglans regia L.) decline in Turkey

Surveys of canker diseases in 26 walnut orchards were conducted in the southeastern Turkish provinces of Batman, Diyarbakir, and Sanliurfa in 2020. Decline symptoms on trees were most severe in the trunks of grafted trees grown on unproductive local rootstocks with Chandler and Franquette scions throughout the surveyed areas, and samples were taken from those trees. Lasiodiplodia theobromae was identified from the cankers on the trees of the seven orchards, while Neoscytalidium dimidiatum was identified from the other 19 orchards based on morphological characteristics. DNA sequencing analysis of 10 representative isolates using the ITS1-5.8S-ITS2 (ITS), the large subunit (LSU), and the translation elongation factor 1-alpha (EF-1 alpha) genes confirmed the identification of the causal fungi. Pathogenicity tests on 2 year-old Chandler seedlings revealed that L. theobromae and N. dimidiatum produced lesion length averages of 7-8 cm and 15-25 cm after 3 weeks of stem-inoculation, respectively. Koch's postulates were confirmed by successfully reisolating the fungi only from plants inoculated. This is the first study to document L. theobromae as the causal agent of walnut tree decline and cankers in Turkey.

Automated summary

Surveys were conducted in 26 walnut orchards in southeastern Turkish provinces in 2020. The decline symptoms were severe on grafted trees with unproductive local rootstocks and Chandler and Franquette scions. Lasiodiplodia theobromae and Neoscytalidium dimidiatum were identified as the causal fungi. Pathogenicity tests showed that L. theobromae and N. dimidiatum produced lesions on Chandler seedlings.