Genetic analysis of powdery mildew disease in Turkish hazelnut

In the last 2-3 years, a new and damaging form of powdery mildew disease has become widespread in hazelnut growing areas in Turkey. The causative agent of this disease has been determined to be the fungal pathogen Erysiphe corylacearum (Sezer et al., 2017). To ensure the sustainability of Turkish hazelnut production, our aims are twofold: i) to analyze the genetic diversity of the fungal pathogen, and ii) to discover hazelnut genes that confer resistance to the new disease. We carried out a survey of infected hazelnut orchards in Trabzon, Giresun and Ordu in 2016, and characterized the fungi found in these orchards using the DNA barcoding approach for two variable regions of the ribosomal RNA gene. Notably, all of the E. corylacearum isolates tested had identical sequences in these regions. Furthermore, SSR markers were developed from 2 previously sequenced Erysiphe genomes, and in the E. corylacearum isolates, no genetic variation was observed at any of 6 putative SSR loci, consistent with the current outbreak originating from a single recent transmission event. As a foundation for identifying potential resistance genes, we have identified a number of individuals from the genetic resources orchard at Giresun Hazelnut Research Institute that are resistant to the mildew disease caused by E. corylacearum. We plan to use these resistant hazelnut individuals in order to identify candidate genes that could be used in future hazelnut breeding programmes.