Evaluation of fungicide resistance in alternaria populations affecting cruciferous crops

Abstract:

Alternaria leaf spot is among the most yield-limiting foliar diseases of cruciferous crops in southern Thailand, and repeated fungicide applications have raised concerns about resistance development in pathogen populations. This research surveyed fungicide sensitivity in 93 Alternaria brassicicola isolates collected from cabbage, cauliflower, and Chinese kale fields across five sites in southern Thailand during 2022-2023. Sensitivity to five fungicides representing four mode-of-action groups was tested using mycelial growth inhibition assays on amended PDA. Resistance to DMI fungicides (difenoconazole) was found in 38.7% of isolates, while QoI resistance (azoxystrobin) occurred in 28.0%. Multi-site fungicides mancozeb and chlorothalonil retained full activity with resistance factors below 1.5. Commercial production sites had the highest resistance frequencies, while research station isolates were mostly sensitive. Cross-resistance between DMI and QoI compounds was detected in 16.1% of isolates. These results indicate that single-site fungicide resistance is already established in Alternaria populations of southern Thailand and that multi-site protectants should be included in spray programmes to manage the problem.