Phenotypic and Genotypic Diversity of Ascochyta fabae Populations in Southern Australia

Abstract

Ascochyta fabae Speg. is a serious foliar fungal disease of faba bean and a constraint to production worldwide. This study investigated the phenotypic and genotypic diversity of the A. fabae pathogen population in southern Australia and the pathogenic variability of the population was examined on a differential set of faba bean cultivars. The host set was inoculated with 154 A. fabae isolates collected from 2015 to 2018 and a range of disease reactions from high to low aggressiveness was observed. Eighty percent of isolates collected from 2015 to 2018 were categorized as pathogenicity group (PG) PG-2 (pathogenic on Farah) and were detected in every region in each year of collection. Four percent of isolates were non-pathogenic on Farah and designated as PG-1. A small group of isolates (16%) were pathogenic on the most resistant differential cultivars, PBA Samira or Nura, and these isolates were designated PG-3. Mating types of 311 isolates collected between 1991 and 2018 were determined and showed an equal ratio of MAT1–1 and MAT1–2 in the southern Australian population. The genetic diversity and population structure of 305 isolates were examined using DArTseq genotyping, and results suggest no association of genotype with any of the population descriptors viz.: collection year, region, host cultivar, mating type, or PG. A Genome-Wide Association Study (GWAS) was performed to assess genetic association with pathogenicity traits and a significant trait-associated genomic locus for disease in Farah AR and PBA Zahra, and PG was revealed. The high frequency of mating of A. fabae indicated by the wide distribution of the two mating types means changes to virulence genes would be quickly distributed to other genotypes. Continued monitoring of the A. fabae pathogen population through pathogenicity testing will be important to identify any increases in aggressiveness or emergence of novel PGs. GWAS and future genetic studies using biparental mating populations could be useful for identifying virulence genes responsible for the observed changes in pathogenicity.

Didymella fabae Punith.: mating type occurrence, distribution and phenotyping of the anamorph Ascochyta fabae Speg. in Tunisia

“…(2015) in their study on Syrian populations. In contrast, Blake et al. (2022) reported an equal ratio of MAT1–1 and MAT1–2 in a collection of 311 isolates of A. fabae from South Australia.…”

“…(2015) showed that 1:1 ratio could not be rejected for two populations of A. fabae from Syria although a random mating hypothesis was rejected in these populations. In contrast, a study conducted on 311 isolates of A. fabae collected between 1991 and 2018 in South Australia by Blake et al. (2022) showed an equal ratio of MAT1–1 and MAT1–2.…”

“…The pathogenicity variation of A. fabae has been reported in numerous studies ( Hanounik and Robertson, 1989 ; Maurin, 1989 ; Rashid et al., 1991 ; Onfroy et al, 1999 ; Kharrat et al., 2000 ; Omri Ben Youssef et al, 2016 ; Blake et al., 2022 ). All these studies showed significant different interactions of faba bean lines when inoculated with different isolates suggesting significant pathogenic variability of the fungus and the presence of physiological specialization ( Tivoli et al., 2006 ).…”

“…The D. fabae teleomorph has been reported in many countries: Canada ( Kharbanda and Bernier, 1980 ), Poland ( Filipowicz, 1983 ), United Kingdom ( Jellis and Punithalingam, 1991 ), Australia ( Kaiser, 1997 ), Syria ( Bayaa and Kabbabeh, 2000 ), Spain ( Rubiales and Trapero-Casas, 2002 ) and more recently in Tunisia ( Omri Ben Youssef et al., 2012 ). Since the discovery of the teleomorph form in Tunisia, several questions arose regarding the spatial distribution of these mating types and their respective prevalence within populations ( Ozkilinc et al., 2015 ; Omri Ben Youssef et al., 2019 ; Blake et al., 2022 ). Our study showed that MAT1-2 was more common than MAT1-1 in three Tunisian locations (Bizerte, Jendouba and Kef), and for two locations (Beja and Tunis-Cap Bon) sexual reproduction was regularly observed.…”

Application of the detached-leaf technique to evaluate the pathogenicity of isolates of fungi Ascochyta fabae and Botrytis fabae to faba bean

“…Therefore, some researchers have suggested the existence of races of the pathogen, but this is not fully documented (Díaz-Ruiz et al, 2009;Tivoli et al, 2006). Rather, in the A. fabae population, pathogenicity groups are identified according to the severity of disease symptoms caused by isolates on a set of cultivars with different resistance (Blake et al, 2022). Sexual reproduction allows the emergence of new combinations of virulence, which is associated with the possibility that the pathogen responds to the selection exerted by the introduction of host resistance genes (Ozkilinc et al, 2015;Rubiales and Trapero-Casas, 2002).…”