Identification, pathogenicity and cell wall degrading enzymes of Alternaria spp. associated with grape bunch rot in Iran

Abstract

Grapes have high economic importance and are recognized as one of the key plant species cultivated across the globe, including Iran. However, this plant is sensitive to a wide range of pathogens. Bunch rot disease, caused by Alternaria spp. Stands out as one of the most critical and damaging grape diseases, worldwide. This disease has significant economic losses for producers, every year. In addition, the toxic metabolites produced by this fungus pose a serious threat to fruit yield, marketability, human and animal health. However, identification of Alternaria species causing the bunch rot disease has not been done in Iran, and no grape cultivars with complete resistance to this destructive disease have been identified, so far. Thus, identifying virulence factors of the pathogens, analyzing the resistance levels in grapevines, and examining the plant's defense responses to the disease are essential steps in creating novel and effective strategies for disease management. Therefore, the objectives of this study were to identify Alternaria species responsible for bunch rot disease, to assess pathogenicity levels and various virulence factors associated with the fungal pathogens, and to determine resistance level of various grape cultivars against Alternaria spp. Obtained from this plant in Iran. The samples were collected from grape berries exhibiting typical symptoms of the disease from vineyards in Khorasan-Razavi province of Iran. Morphological identification of the fungi was performed on potato carrot agar (PCA) medium. Three species of Alternaria were identified, including A. alternata, A. tenuissima, and A. destruens. For molecular identification, the ITS1 and ITS4 primers were utilized, followed by sequence analysis. Alternaria alternata was found to be the most prevalent species, while A. destruens was the least frequently obtained species. Three grape cultivars were examined to investigate pathogenicity of the fungal isolates and evaluate resistance levels of the host cultivars. Among the cultivars tested, Sahebi exhibited the highest resistance, while Fakhri was the most susceptible cultivar. The research revealed different degrees of virulence and aggressiveness among the fungal isolates. Isolates of A. alternata and A. destruens showed the highest pathogenicity levels. Analysis of cell wall degrading enzymes revealed that the isolates with higher pathogenicity had higher enzyme activity. This study is the first report of Alternaria spp. Responsible for bunch rot disease in Iran. Also, this is the first global report of A. destruens pathogenic on grapes and exploration of the relationship between pathogenicity factors and pathogenesis of Alternaria spp. Affecting grapes.