Sensitivity of Isolates of Phytophthora infestans to the Fungicides Dimethomorph, Mandipropamid and Propamocarb Hydrochloride

Chemical control of late blight ( Phytophthora infestans ) of potato ( Solanum tuberosum L.) requires a large number of sprays and increases the risk of reduced sensitivity of the oomycete to fungicides. The aim of this study was therefore to assess the sensitivity of isolates of P. infestans from southern Brazil to dimethomorph (DM), mandipropamid (MD) and propamocarb hydrochloride (PH) and to verify whether there is a correlation between the EC 50 values (50% effective concentrations) of the fungicides DM and MD. We selected 43 monosporangial isolates for DM, 47 for MD and 43 isolates for PH, and found the EC 50 values for mycelial growth inhibition for DM to be between 0.086 and 0.218 μg/mL, for MD between 0.008 and 0.037 μg/mL, and for PH in the range of 4.494 to > 100.00 μg/mL. Furthermore, despite the high sensitivity, it was possible to identify a moderate level of positive correlation ( r = 0.43, p = 0.0178, n = 30) in sensitivity in the isolates tested for DM and MD. Finally, the results indicated a wide variation in the sensitivity of the P. infestans isolates to the fungicide PH. These data reinforce the importance of continuous monitoring of fungicide efficacy to guide integrated management strategies and prevent the development of resistance.