A. Solar¹, F. Stampar², R. Veberic² and S. Trdan^3
1 University of Ljubljana, Biotechnical Faculty, Department of Agronomy, Experimental Field for Nut Crops, Maribor, Slovenia
² University of Ljubljana, Biotechnical Faculty, Department of Agronomy, Chair for Fruit, Wine and Vegetable Growing, Ljubljana, Slovenia
³ University of Ljubljana, Biotechnical Faculty, Department of Agronomy, Chair of Phytomedicine, Agricultural Engineering, Crop Production, Pasture and Grassland Management, Ljubljana, Slovenia

SUMMARY Introduction – Since the occurrence of walnut husk fly (WHF) is associated with significant yield and nut quality losses, a detailed information on cultivars’ susceptibility is essential for profitable production of common walnut. Materials and methods – Seven pomological traits related to external nut quality and five traits describing kernel quality were observed in 10 commercial cultivars to determine WHF-damage level. Results and discussion – The damage was cultivar- and infestation time-related. It was shown by reduced nut weight within a range from 0.1 g (‘Elit’) to 1.9 g (‘Franquette’), by an increased number of baby nuts from 1.4% (‘G-139’) to 18.1% (‘Chandler’) and by a higher ratio of empty nuts ranging from 0% (‘G-139’) to 12.5% (‘Fernette’). Internal damage was represented as kernel weight reduction within a range of 0.1 g (‘A-117’) to 1.2 g (‘Franquette’) and as reduced kernel percentage from 0.2% (‘G-139’) to 7.0% (‘Chandler’). Between 5.3% (‘A-117’) and 50.0% (‘Chandler’) of the kernels had more than a half of their surface darkened, and totally black kernels appeared within a range of 4.0% (‘A-117’) to 36.1% (‘Chandler’). Early WHF infestation caused shriveled kernels, which varied from 0% in ‘G-139’ to 30.6% in ‘Cisco’ and ‘Chandler’ cultivars, together with kernel mouldiness, ranging from 37% (‘Fernette’) up to 75% (‘Elit’) mould. Infested ‘Fernor’ preserved the best external and internal quality, while ‘Franquette’ and ‘Chandler’ showed the highest WHF damage levels. Conclusion – The results may be taken into consideration when planting new walnut orchards, and suggest favouring less susceptible cultivars. Keywords cultivars, WHF damage, Juglans regia L., Rhagoletis completa Cresson, Diptera: Tephritidae

Significance of this study What is already known on this subject? Walnuts are nutritionally valuable food with many health beneficial effects. Their production is strongly affected by walnut husk fly (WHF). Significant crop losses due to the pest is recognized world-wide. Detailed information on the infestation impact on external and internal nut quality of cultivars grown in continental climate is still insufficient. What are the new findings? Due to the WHF infestation, both in-shell walnut and kernel quality was affected, showing as reduced nut and kernel weight and kernel percentage, empty nuts, dark shells and kernels as well as shriveled and mouldy kernels. The damage was shown as cultivar-specific and related to the time of WHF infestation. What is the expected impact on horticulture? An information on natural resistance/tolerance of important commercially grown cultivars to WHF is provided for continental climatic conditions. Since efficiency of different plant protection methods is limited, the cultivar choice can present one of the most promising preventive methods for controlling the pest in question.

E-mail: anita.solar@bf.uni-lj.si  

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Received: 22 August 2018 | Accepted: 28 November 2018 | Published: 10 March 2020 | Available online: 10 March 2020