Abstract Description

Potato (Solanum tuberosum) is a globally important crop, however, its production is increasingly threatened by Alternaria species, the causal agents of early blight and brown spot. Fungicides are widely used to manage this disease, but their efficacy is increasingly threatened by resistance development. This study aimed to evaluate the sensitivity of Alternaria alternata, A. arborescens, A. solani and A. grandis isolates from potato in South Africa to quinone outside inhibitors (QoI), demethylation inhibitors (DMI) and succinate dehydrogenase inhibitors (SDHI) fungicides, and to identify resistance-associated mutations through whole-genome sequencing. Isolates previously tested in vitro were sequenced, and mutations were cross-referenced against known resistance loci. In the cytochrome b gene, a G143A mutation was detected in A. alternata and A. arborescens, while F129L was present in A. solani and A. grandis. The cyp51 gene carried the G462S mutation in all species except one A. solani isolate, which harboured a G153C mutation. Mutations associated with SDHI resistance were diverse: A. alternata carried I279V, H134R, S73L, A47T, and E177K mutations, whereas A. arborescens carried I279V, D36N and H134R mutations. In A. solani, I280V was present in 87.5% of isolates, and in A. grandis in 25%, with both species carrying H134R in SdhC but no SdhD mutations. These findings confirm the presence of multiple resistance-associated mutations in South African Alternaria populations and highlight the need for ongoing monitoring to guide sustainable fungicide use in potato production.