Osetljivost Cercospora beticola (Sacc.) prouzrokovača lišća šećerne repe prema fungicidima / Sensitivity of Cercospora beticola (Sacc.) the causer of sugar beet leaf spot to fungicides

Cercospora beticola, causal agent of sugar beet leaf spot, is economically the most significant sugar beet leaf pathogen, primarily controlled by fungicides. In Serbia, fungicides from groups of benzimidazoles, DMIs and strobilurins have been widely used. Since these fungicide groups have a site-specific mode of action, there is a high risk for resistance development in target organisms, which is the most important limiting factor of CLS chemical control. One hundred and three single-conidia isolates were collected from 60 representative localities in leading sugar beet production region in Serbia. Five isolates causing leaf spot on chard and beetroot from localities on which fungicides were never applied, were tested as a referent population for baseline sensitivity. Based on EC50 values of referent isolates, discriminatory concentrations for each tested fungicide were established: carbendazim 5 µg/ml, flutriafol 1.25 µg/ml, tetraconazole 0.6 µg/ml and azoxystrobin 0.1 µg/ml. Resistance to carbendazim was detected in 96% of isolates, whereas frequency of resistant isolates to triazole fungicides varied from 9.2 % to 16.3 % for tetraconazole and flutriafol, respectively. All isolates resistant to triazoles were resistant to carbendazim as well which, to our knowledge, is the first report of double resistance to fungicides in C. beticola. Sensitivity to azoxystrobin remained unchanged since none of tested isolates developed on fungicide amended medium. Detection of resistant isolates to flutriafol and carbendazim using CAPS markers confirmed the results of the in vitro tests. The efficacy of carbendazim, flutriafol, azoxystrobin and tetraconazole at commercially recommended dose was evaluated in field trials where sugar beet plants were inoculated with isolates sensitive and resistant to flutriafol and carbendazim. Carbendazim and flutriafol efficacy was very low in plots inoculated with isolates resistant to these fungicides, while they were efficient control of disease caused by sensitive isolates. Azoxystrobin and tetraconazole showed good efficacy in disease control regardless of the isolate sensitivity. Presented results will contribute to the development of pathogen population sensitivity monitoring strategy which could be used for effective CLS management in the region. Also, identification of decreased sensitivity of C. beticola to benzimidazoles and triazoles can help researchers in examining the potential of different fungicide resistance management practices, as well as in selection of fungicides to be used in areas where decreased sensitivity or resistance occurred.