Avirulence (Avr) genes in fungal pathogen Venturia inaequalis, a causal agent of scab disease on apple trees

Apple scab is one of the most commercially significant apple diseases, caused by the ascomycete fungus Venturia inaequalis. In gene-for-gene (R-Avr) interactions, a subset of effectors known as avirulence (Avr) gene products can be identified by the host, resulting in the activation of the host defence response. Several important scab resistance (R) genes have been identified in apple cultivars and wild relatives; however, little is known about interacting Avr genes from V. inaequalis. Major Avr genes such as AvrRvi1, AvrRvi2, and AvrRvi5 have been extensively studied in this fungus. A few complicated interactions involving AvrRvi2 AvrRvi8 and AvrRvi9 have also been discovered. Several Avr analogues have been predicted from this fungus using comparative genomics techniques. Therefore, the aim of this study is to characterize several Avr genes in V. inaequalis to better understand host-pathogen interactions in this pathological system. This review provides an up-to-date understanding of the large number of Avr genes identified to date in V. inaequalis.