Hazelnut Breeding

Hazelnut (Corylus spp.), also known as filbert, is an important temperate nut that has been used as a food crop since ancient times. There is a tremendous amount of genetic diversity in Corylus and more than 500 Corylus avellana cultivars have been described in the literature. World production is based on fewer than 20 ancient C. avellana cultivars that originated with the selection and clonal propagation of superior plants primarily in the Mediterranean Basin and along the Black Sea coast. Large nuts are sold in-shell, while the majority of the world’s hazelnuts are sold as kernels for use by the confectionary industry. In Turkey, the most important producing country, nut clusters are picked from the tree by hand, often in orchards on very steep terrain. The husks of Turkish cultivars are long and clasping and retain the clusters on the tree for harvest. In other countries, nuts are mechanically harvested from the ground, and nuts that fall free of the husk at maturity are desired. As newer production systems are adopted, including high-density orchards of compact trees, appropriate cultivars, management practices, and machinery will be needed. Because of high demand for kernels, new orchards are being planted in many countries, including areas where the climate is less than ideal for the species. Interspecific hybridization offers an opportunity to expand production to new areas. Hazelnut has many arthropod pests, of which big bud mite (primarily Phytoptus avellanae) is a serious one. Many diseases affect hazelnut, of which eastern filbert blight (EFB) caused by the fungus Anisogramma anomala is the most severe threat in North America. Genetic resistance to big bud mite and EFB has been identified and incorporated into new cultivars. Early genetic improvement efforts were largely by nurseries and growers. Over the past 60 years, 11 breeding programs have been active in different countries, but today there are only two relatively large programs, both in the USA. Standard methods to breed hazelnuts have been very effective although it takes 17 years from the time a controlled cross is made until a new cultivar is released. Biotechnology has added tools and knowledge to genetic improvement efforts. Thousands of DNA markers have been developed and scored, linkage maps have been constructed, and our knowledge of genetic diversity and the control of traits in Corylus have improved. To date, however, the use of markerassisted selection has been limited to EFB resistance from ‘Gasaway’. Genome sequences have been assembled into chromosome-length scaffolds and genes have been annotated, allowing efficient marker development for genetic studies and marker-aided breeding. The use of genome editing is limited by an inability to regenerate whole plants from single cells. The huge amount of genetic variability in C. avellana and other species can be exploited to develop improved cultivars with wider adaptation. Worldwide demand for hazelnuts continues to increase. With the proven effectiveness of traditional plant breeding approaches, the diverse genetic resources that have been collected and preserved, and the considerable promise of modern genetic and genomic tools, the future looks very bright for further genetic improvement of hazelnut.