Tank-mix partners selectively enhance DMI fungicides in controlling Cercospora leaf spot and impact the genetic diversity of a Cercospora beticola population.
Cercospora leaf spot (CLS) caused by Cercospora beticola is considered the most destructive foliar disease of sugarbeet worldwide. With limited availability of commercial sugarbeet varieties having strong tolerance to CLS, the use of fungicides remain critical for managing CLS and reduce the extent of economic losses. Demethylation inhibitor (DMI) fungicides are currently the most important group of fungicides used to manage CLS in sugar beets. Additionally, combining DMI fungicides with a broad-spectrum tank-mix partner is highly recommended to delay the development of fungicide resistance. A two-year field study (2020 and 2021, artificially inoculated) was conducted to evaluate the efficacy of DMI fungicides with and without tank-mix partners for managing CLS. A 5 x 7 factorial split-plot design with four replications was used. Treatments included four DMI fungicides (prothioconazole, tetraconazole, difenoconazole + propiconazole, and mefentrifluconazole) and six tank-mix partners (mancozeb, copper oxychloride + copper hydroxide, sodium bicarbonate, sulfur, potassium phosphite, and Bacillus subtilis). CLS disease severity was quantified using APS Assess 2.0 software. In both years, significant differences were observed for the tank-mix partners with mancozeb, copper, potassium phosphite, and sulfur resulting in increased recoverable sucrose compared to DMI fungicides alone. Interactions among DMI fungicides and tank-mix partners for CLS severity were observed. In addition to the field trial, single-conidia isolates were collected, representing each treatment before and after fungicide applications. The genetic diversity of isolates was evaluated with 8 fluorescent-tagged microsatellite markers. An estimated 306 original multilocus genotypes (MLGs) were observed among 763 isolates, with 93 MLGs identified before fungicide applications and 250 MLGs identified following exposure to fungicide treatments. Interestingly, only 37 MLGs were observed in both collection periods. The results from this field study will help formulate effective and practical fungicide-based recommendations for managing CLS and understand the impact of fungicides on C. beticola genetic diversity.