Genetic structure analysis and assessment of susceptibility to Fusarium euwallaceae of popular avocado cultivars

Abstract

Avocado is an important economical crop, with breeding cycles of more than 20 years to produce improved individuals. The utilisation of molecular marker technologies can aid in speeding up breeding cycles by providing insights into the mechanisms underlying improved performance against biotic and abiotic stressors. An emerging biotic threat to the avocado industry is the Polyphagous Shot Hole Borer (PSHB) and its fungal symbiont Fusarium euwallaceae. Currently there is scarce information available regarding the resistance capacity of local cultivars to this pest-pathogen complex. Additionally, no genetic marker information or genetic population studies have been performed on germplasms available in South Africa, thus, hindering the selection of individuals with valuable characteristics.

Chapter 1 of this thesis presents a review of the literature on the study of avocado populations using molecular markers and the emergence of new threats to the industry in South Africa. In this review, the basics of avocado are discussed including the origin of available varieties, their propagation and the current breeding methods used in the selection of improved genotypes. This is followed by a discussion of using molecular markers in avocado population management. Lastly, the common pathogens and pests affecting avocado and the emerging threat of the PSHB and its fungal symbiont F. euwallaceae to the South African avocado industry are discussed. .

Chapter 2 evaluates an avocado fruiting cultivar breeding population in South Africa using single nucleotide polymorphism (SNPs) markers. This evaluation includes the identification of mislabelled samples, as well as, the validation of clonal material. The population was also assessed to verify horticultural variety and determine genetic diversity and population structure.

Chapter 3 involves the assessment of popular avocado cultivars’ susceptibility towards F. euwallaceae, firstly, using a detached branch pathogenicity trial involving lesion measurements, and secondly, using a plantlet pathogenicity trial involving a real-time Polymerase Chain Reaction (PCR) approach to assign susceptibility or tolerance status.

Chapter 4 reflects on the outcomes of this project and includes recommendations that may be of interest for future research.