Sensitivity of Argentinean isolates of Leptosphaeria maculans to Azoxystrobin, Boscalid and Prothioconazole

Blackleg, caused by Leptosphaeria maculans, is a major disease affecting oilseed rape (Brassica napus) worldwide. The use of fungicides is an essential tool for disease management; however, limited information is available regarding L. maculans sensitivity to fungicides in Argentina. In this study, the sensitivity of L. maculans isolates collected from five oilseed rape-growing regions in Argentina was evaluated against three fungicides with different modes of action: prothioconazole (DMI), azoxystrobin (QoI), and boscalid (SDHI). The effective concentration required to inhibit mycelial growth by 50% (EC50) was determined using in vitro assays. Mean EC50 values for prothioconazole, azoxystrobin, and boscalid were 0.1545 {micro}g mL-1, 0.0349 {micro}g mL-1, and 0.1557 {micro}g mL-1, respectively. A wider variability in sensitivity was observed for azoxystrobin and boscalid compared to prothioconazole. Additionally, discriminatory dose assays revealed significant differences in growth inhibition among isolates, with some showing reduced sensitivity. Spearman correlation analysis indicated a significant positive association between azoxystrobin and boscalid sensitivity at both low and high doses, suggesting potential cross-resistance. However, no significant correlations were found between prothioconazole and the other fungicides. Furthermore, isolates with reduced sensitivity (RSI < 1) did not exhibit a consistent cross-resistance pattern between QoI and SDHI fungicides, indicating that the observed correlations may be influenced by the overall distribution of sensitive isolates rather than a shared resistance mechanism. These findings provide the first insights into L. maculans fungicide sensitivity in Argentina, highlighting the need for continued monitoring to prevent resistance development and ensure effective blackleg management.