Fitness and Management of Site-specific Fungicide Resistant Cercospora beticola Isolates from Sugar Beet

Abstract

Cercospora leaf spot (CLS), caused by the fungal pathogen Cercospora beticola, is one of the most destructive foliar diseases on sugar beet. It severely affects productivity and profitability of the sugar beet industry. CLS is managed by using resistant cultivars, rotating with non-host crops, and applying effective fungicides in a timely manner. In North Dakota and Minnesota, site-specific fungicides, such as quinone outside inhibitors (QoIs) and demethylation inhibitors (DMIs), had been widely and extensively applied to control C. beticola which has developed resistances to these fungicides. The mycelial growth, spore production, spore germination, and aggressiveness of QoI and/or DMI resistant isolates were compared to sensitive isolates in a laboratory and greenhouse study. Results indicated that the QoI and/or DMI resistant isolates had a relatively slower disease development on sugar beet leaves due to their fitness penalty in sporulation and mycelial growth but still caused high disease severities as sensitive isolates. Fungicides were evaluated to manage the QoI and/or DMI resistant C. beticola isolates in a sensitivity study and a greenhouse and field study. Copper-based multisite activity fungicides were evaluated for controlling fungicide resistant C. beticola. The mean EC50 values for nine copper-based chemicals ranged from 1 to 10 ug/ml using a spore germination assay. In a greenhouse study, Fertileader (a copper-based fertilizer) caused leaf injury and was not evaluated, but the other tested chemicals provided significantly better control of CLS compared to the control check with Badge X2, Champion, Cuprofix, COCS and Ridomil having significantly small AUDPC. Newer site specific fungicides with different modes of action that have never been used for CLS management in North Dakota and Minnesota were also tested. The mean EC50 values were 4.9, 33.1, 99.4, and 481.6 ug/ml using mycelial growth assay and 5.7, 4.1, 9.2, and 4.2 ug/ml using spore germination assay for cyprodinil, fluazinam, pydiflumetofen, and Chlorothalonil, respectively. In a two-year field study, all the fungicides resulted in significantly better disease control, significantly higher beet tonnage, and recovered sucrose compared to the nontreated check. However, none of tested fungicide treatments performed better than the industry’s standard fungicide, triphenyltin hydroxide.