A strategy for controlling Cercospora leaf spot, caused by Cercospora beticola, by combining induced host resistance and chemical pathogen control and its implications for sugar beet yield
2022
Morsy, Saad Z. | Shady, Mohamed F. | Gouda, Moustafa I. | Kamereldawla, Bahnasy A. | Abdelrazek, Mohamed A. S.
Cercospora beticola populations resistant to methyl benzimidazole carbamate (MBC), quinone outside inhibitor (QoI), and demethylation inhibitor (DMI) fungicides have been reported in Egypt. This pathogen causes a difficult to control variant of Cercospora leaf spot (CLS) on sugar beet. We conducted field experiments over two years to investigate treatment efficacies on MBC- and QoI-resistant, but partially DMI-sensitive, C. beticola from a research farm in Sakha, Egypt. DMI fungicides showed moderate efficacy with half the maximal effective concentration (EC₅₀). Efficacy was 67.2–69.1% and 63.4–63.6% for epoxiconazole (EPO) and propiconazole (PRO), respectively. When each fungicide was mixed with salicylic acid (SA) at EC₅₀ (EPO + SA) and EC₅₀ (PRO +SA), efficacy was 77.5–79.1% and 77.0–78.2%, respectively. The QoI fungicide, azoxystrobin, showed lower efficacy (46.4–49.5%) when applied alone, but enhanced efficacy when combined with SA at EC₅₀/EC₅₀ (77.0–78.2%). Carbendazim alone was only 47.5–45.1% effective, but when mixed with SA, had a 67.1% co-toxicity factor in vitro, indicating antagonism between components. An Artemisia cina aqueous extract (WEA) combined with SA (WEA/SA) showed 76.0–78.2% efficacy, compared with 69.0–71.0% and 49.0–52.7% for WEA and SA, respectively. Single treatments were classified into five sensitivity classes from sensitive (<1) to resistant (>100) based on their EC₅₀ values. Significant increases in insensitivity were observed from 2015/16 to 2017/18. All treatments increased total yield, sucrose percentage, and recoverable white sugar compared with the untreated control. A combination of control strategies could allow for effective disease and fungicide resistance management.
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