Characterization of Alternaria species associated with Alternaria leaf blight of mustard reveals the genetic toxigenic potential of Alternaria alternata

Abstract

Alternaria leaf blight (ALB), caused by the Alternaria species complex, is an economic threat to mustard cultivation all over the world. In this study, 700 infected leaf samples were collected from 28 blocks across six districts of the Bundelkhand region, Uttar Pradesh, India. To assess the prevalence and distribution of Alternaria species, 28 representative isolates were selected for further study. Morphological traits separated the majority of the Alternaria isolates into three morphotypes corresponding to A. brassicae, A. brassicicola, and A. alternata, which cause leaf blight. The pathogenicity test revealed that 18 isolates were highly virulent, 6 isolates were moderately virulent, and 4 were less virulent. Molecular analyses of the 18 highly virulent isolates, based on the internal transcribed spacer (ITS) region and the Alternaria-specific Alt a1 marker, confirmed the presence of A. brassicae (6 isolates), A. brassicicola (3 isolates), and A. alternata (9 isolates) in the region. Haplotype network analysis of concatenated ITS and Alt a1 sequences revealed two haplotypes among the A. alternata isolates and one haplotype each among the A. brassicae and A. brassicicola isolates. Additionally, all virulent A. alternata isolates carried the polyketide synthase genes (pksH and pksJ), indicating their genetic potential for Alternaria toxin biosynthesis. The predominance of A. alternata in mustard crops and the presence of toxin-biosynthesis-associated genes may pose a threat to edible oil production and safety, thereby raising serious concerns and warranting further investigation of toxin production. These findings highlighted the need for targeted resistance strategies against the prevalent Alternaria species in the region.