P-311 - Impact of sugar beet variety mixtures on the epidemiology of Cercospora leaf spot

Cercospora leaf spot (CLS), caused by Cercospora beticola, remains a major threat to sugarbeet production. The recent widescale adoption of varieties with the BvCR4 resistance gene raises concerns about pathogen adaptation. Variety mixtures with varying CLS tolerance may help manage CLS, but their effects on disease progression and resistance durability remain unclear. We evaluated the impact of variety mixtures and a reduced fungicide program on CLS severity, sucrose yield, and C. beticola population dynamics. A field trial was conducted in 2024 in northwest Minnesota using a split-plot design with four replications. Main plots included a BvCR4-resistant (CR+) and a moderately susceptible (MS) variety planted in pure stands or mixtures (25:75, 50:50, 75:25). Subplots received either no fungicide or a reduced fungicide three spray program. CLS severity was assessed weekly. Population genetics analysis was performed on 320 C. beticola isolates using eight microsatellite markers. Mid-season CLS severity was significantly lower in mixtures with ≥50% CR+, even without fungicides. Significant variety-by-fungicide interactions were observed for final CLS severity and sucrose yield. Population analysis revealed two dominant multilocus genotypes (MLGs) regardless of mixture. Taken together, variety mixtures can delay CLS progression as well as outperform pure MS stands in yield. Population divergence suggests potential pathogen adaptation, emphasizing the need for integrated CLS resistance management.