DNA barcoding, aggressiveness of Bipolaris sorokiniana isolates, and pathogenicity of emerging B. gossypina in barley in subtropical southern Brazil

Southern Brazil has witnessed significant outbreaks of leaf blotch disease in barley over the last decade, leading to substantial losses in grain yield and quality. The increasing importance of this disease is mainly attributed to the shift towards cultivars with higher grain yields and superior malting quality, but they are more susceptible to leaf blotch. A field survey was conducted in the major barley-producing areas of Paraná state using DNA barcoding techniques to identify Bipolaris sorokiniana isolates, the causative agent of spot blotch, aiming to determine the extent of pathogenic variability among the isolates. DNA barcoding and phylogenetic analyses were based on internal transcribed spacers 1 and 2 with the 5.8S region (ITS) of rDNA, glyceraldehyde-3-phosphate dehydrogenase (gapdh), and translation elongation factor 1-α (tef1) genes. Out of the 124 isolates examined, which were collected from 20 commercial barley fields each year in 2020 and 2021, 116 isolates were identified as B. sorokiniana and eight isolates as B. gossypina. Koch's postulates confirmed the pathogenicity of B. gossypina in barley, representing a novel occurrence worldwide. The fungus causes the development of elongated brown lesions surrounded by irregular yellow halos, starting at minute points. Subtle differences between these symptoms and spot blotch caused by B. sorokiniana are discussed. This study also assessed the aggressiveness of 16 B. sorokiniana isolates on potted barley grown under controlled conditions, using a visual infection rate (IR) scale ranging from 1 to 9. Significant differences in aggressiveness were recorded among the isolates, with the IR ranging from 5.1 to 7.4 in the cultivar ANA03 and 5.7 to 8.1 in the cultivar Imperatriz. The interaction between the isolates and cultivars was not significant. These findings could support breeding programs aiming to develop cultivars with genetic resistance to spot blotch disease in Brazil.