Inheritance of resistance to chocolate spot (Botrytis fabae) in faba bean (Vicia faba)

Abstract

Faba bean (Vicia faba L.) is an important cool season legume species belonging to the Fabaceae and is grown widely in the world due to its ecological and environmental value. However, its production has been constrained by biotic and abiotic stresses, which includes chocolate spot caused by the fungus Botrytis fabae. The disease is one of the major contributors to low productivity of faba bean resulting in significant yield losses. Understanding the genetics of chocolate spot resistance is instrumental for developing efficient resistance breeding strategies. The objectives of this study were to: i) characterize the pathogenic variability of B. fabae isolates collected from western Canada, ii) to screen Chinese faba bean germplasm for resistance to chocolate spot using three different screening methods, iii) to identify QTLs associated with chocolate spot resistance in a geographically and genetically diverse genome wide association mapping (GWAS) panel, iv) to identify potential candidate genes which are involved in chocolate spot resistance. Results from the pathogenic variability study using 15 B. fabae isolates and 22 faba bean genotypes revealed variability in aggressiveness among the isolates without the presence of a race structure. Evaluation of Chinese faba bean genotypes as well as other faba bean lines originating from different parts of the world using a detached leaf assay, a whole plant assay and field screening showed that the genotypes had a wide range of resistance to B. fabae. A total of 16 SNPs were significantly associated with chocolate spot resistance based on a genome wide association study (GWAS) using 141 faba bean genotypes evaluated in the phytotron and a semi-open polytunnel. Evaluation of faba bean genotypes under field and controlled conditions showed a continuous distribution in disease severity indicating the presence of quantitative resistance in faba bean. Population structure analysis of the faba bean GWAS panel grouped the genotypes into three subpopulations. The identification of 16 QTLs will facilitate marker-assisted breeding by using the SNPs associated with QTLs as markers. The phytohormone signalling pathways using jasmonic acid, ethylene or salicylic acid as signalling molecules were suppressed for 24 hours post inoculation. Its is hypothesised that B. fabae effectors may have caused the suppression of early resistance responses to facilitate infection.