In-vitro sensitivity of Alternaria solani isolates to azoxystrobin and difenoconazole fungicides in Kenya and detection of Cyt b mutations associated with azoxystrobin resistance. (August 2022)

• Record Type: Journal Article
• Authors: Nuwamanya, Andrew M.; Runo, Steven; Mwangi, Maina
• Is Part Of: Crop protection. Volume 158(2022)
• Publication Date: 2022-08
• Subjects: Azoxystrobin -- Difenoconazole -- Early blight -- Alternaria solani -- Cyt bgene -- F129L -- G143S; Plants, Protection of -- Periodicals; 632.05
• DOI: 10.1016/j.cropro.2022.106010
• Languages: English
• ISSNs: 0261-2194

Abstract

Abstract: Early blight (EB) caused by Alternaria solani is one of the most important tomato diseases in Kenya and farmers predominantly rely on synthetic fungicides to control it. However, there have been many complaints from farmers about the declining control efficacy of many fungicides used. In this study, 96 A. solani isolates, collected from tomato fields in 3 counties of Kenya (Kirinyaga, Kajiado and Kiambu), were assessed in vitro for sensitivity to two commonly used fungicides; Azoxystrobin and Difenoconazole, at varying concentrations. Isolates were considered resistant to the fungicide whenever their Percent Mycelial Growth Inhibition (MGI) at manufacturer's recommended dosage was below 50% and sensitive if above 50%. Although Difenoconazole resistance was not detected at the tested concentrations, majority of the isolates (64%) were resistant to Azoxystrobin. To determine the genetic basis of this Azoxystrobin resistance, DNA was extracted from all isolates, the Cyt b gene amplified and analyzed for resistance-associated mutations, at positions 129, 137 and 143. Results indicated that two mutations were present in the isolates. The F129L mutation was present in all Azoxystrobin resistant isolates plus 10 sensitive ones with MGI values close to the discriminatory threshold (MGI 50%). The G143S mutation was detected in 26 isolates (27%) but did not have a significant effect on sensitivity of isolates to Azoxystrobin. These findings indicate that Kenya's A. solani populations are developing resistance to some fungicides by mutation and provide justification for deployment of anti-resistance strategies, to ensure more effective management of tomato early blight. Highlights: Majority of Alternaria solani isolates were resistant to azoxystrobin. Difenoconazole resistance was not detected at the tested concentrations. Two mutations (F129L and G143S) were detected in the cyt b gene of most isolates. Isolates with the F129L mutation had low sensitivity to azoxystrobin. The G143S mutation did not affect the sensitivity of isolates to azoxystrobin.