Diaporthe as the main cause of hazelnut defects in the Caucasus region

Production of hazelnuts is affected by defects, the incidence of which is year-dependent but also economically damaging. The objectives of the present study were to quantify the incidence of different fungal genera in hazelnuts, and define the causal agent/agents of nut defects. A 4-year study (2013 to 2016) was carried out in the Caucasus region, in an orchard planted in 2008 with hazelnut cv. Anakliuri. Hazelnuts were sampled at early and full ripening stages, observed for defects, and then associated fungi were isolated. Nuts with necrotic spots and internal browning were commonly found, and some collected in 2016 were completely rotted, wet and almost black. Alternaria, Aspergillus, Cladosporium, Colletotrichum, Diaporthe, Fusarium, Penicillium, Pestalotiopsis and Phoma spp. were regularly isolated from diseased and symptomless nuts. Diaporthe spp. was the dominant genus, with increasing incidence from early to full ripening, and were more isolated from defected compared to healthy kernels and in the years with the greatest incidence of defects. Rainfall was associated with the incidence of nut defects. The role of Diaporthe as a key cause hazelnut defects was confirmed by pathogenicity tests. Three isolates from the Diaporthe population were identified as D. eres on the basis of EF, ACT, TUB and ITS loci. This supports the crucial role of climate during the crop-growing season for the development of defects in hazelnuts caused by Diaporthe.