Walnut (Juglans regia L.) is one of the oldest trees with harvestable products
known to humans and has a history dating to 7000 BC in Persia. Walnut breeding
programs aim to release productive scion cultivars with disease resistance and
high-quality nuts, along with rootstocks resistant to biotic and abiotic stresses.
Genetic improvement of walnut began with the selection of superior trees in their
main centers of origin, primarily from the Persian plateau. The first selection and
grafting of superior walnut genotypes began in France. The first organized walnut-breeding
program employing targeted hybridization began in the USA in 1948, primarily
using introduced French cultivars and selected local genotypes derived from
seed imported from centers of origin (Iran, Afghanistan, China). Currently, both
conventional hybridization with phenotypic evaluation and molecular breeding
approaches are used in the USA programs as well as those in France, China, Iran,
Spain and Italy. Recent advances in biotechnology and genomics show potential to
accelerate cultivar development. In addition, the exploration, description, and preservation
of biodiverse germplasm can provide a gene bank of desirable traits and
enable biotechnologists to conduct breeding more accurately and rapidly in the
future. Recent advancements have opened up new avenues to enhance the efficiency
of walnut breeding to release new scions and rootstocks. These include next-generation
sequencing (NGS) techniques, bioinformatics tools, high-throughput
genotyping platforms and genomics-based approaches such as genome wide association
studies (GWAS), marker-assisted selection (MAS), genomic selection (GS)
and genome editing with the CRISPR-Cas9 system. In this chapter, we describe thebackground and development of conventional walnut breeding programs in the
leading walnut producing countries of the USA, France, China, Iran and Turkey,
and finally focus on the current use and status of molecular breeding and biotechn