Resistance of Cercospora beticola Sacc isolates to thiophanate methyl (benzimidazole), demethylation inhibitors and quinone outside inhibitors in Morocco

Sugar beet is a major crop in Morocco and Cercospora leaf spot is one of its most important fungal diseases. In Morocco, thiophanate methyl (benzimidazole) and difenoconazole (demethylation inhibitor, DMI) have been used extensively in the management of Cercospora leaf spot. In this study, samples of Cercospora beticola Sacc were collected from four major production areas. The identification of all isolates was confirmed using a PCR test with specific primers. Radial mycelia growth of each isolate in unmodified potato dextrose agar medium was compared to mycelia growth in the same medium modified with thiophanate methyl (1, 5, 10 and 50 ppm) or the DMIs difenoconazole, epoxiconazole and tetraconazole (0.1, 0.5, 1, 5, 10 and 50 ppm) or the quinone outside inhibitors (QOIs) azoxystrobin and trifloxystrobin (1, 5, 10 and 50 ppm). The percentage of inhibition obtained was used for the half maximal effective concentration (EC₅₀) calculation. All the isolates showed resistance to the thiophanate methyl molecule to different degrees. Three groups were identified: low resistance with EC₅₀ less than 100 ppm, moderate resistance with EC₅₀ between 100 and 1000 ppm, and very resistant with EC₅₀ more than 1000 ppm. For difenoconazole, 41% of isolates were sensitive (EC₅₀ < 0.05 ppm) and 59% were resistant, while for tetraconazole and epoxiconazole 6% were sensitive (EC₅₀ < 0.01 ppm), 66.7 % had medium resistance (0.01 < EC₅₀ < 1 ppm) and 27.3% were resistant (EC₅₀ > 1 ppm). For QOIs, azoxystrobin was less effective for mycelial inhibition than trifloxystrobin, with 54.5% of isolates with resistance to azoxystrobin (EC₅₀ > 100 ppm).