PHYTOPATHOLOGY G.S. Karaoglanidis et al. (2003) Phytoparasitica 31(3):275-282 Lack of Influence of Host Plant Disease Resistance on the Evolution of Resistance to Sterol Demethylation-inhibiting (DMI) Fungicides in Cercospora beticola G.S. Karaoglanidis, *'1 C.C. Thanassoulopoulos I and P.M. Ioannidis 2 Field experiments were conducted during 1997 and 1998 to determine the effects of sugar beet cultivar susceptibility to Cercospora leaf-spot on the sensitivity of Cercospora beticola isolates to the triazole fungicide flutriafol. Four cultivars with different levels of disease resistance were treated in experimental plots with six spray applications of flutriafol. Disease assessments were carried out at 15-day intervals. Sensitivity to flutriafol was measured on isolates collected from the plots ,-~ 15 days after the last flutriafol application. Measurements of disease severity and calculations of AUDPC (area under disease progress curve) values showed a distinct differentiation among cultivars, refecting their level of disease resistance. Disease severity was significantly lower in cvs. 'Bianca' and 'Areth' than in 'Univers' and 'Rizor' both in the untreated and in the flutriafol-treated plots. Fungal isolates from flutriafol- treated plots were less sensitive to the fungicide than were isolates from untreated plots. However, no differences in isolate sensitivity were observed among the cultivars, as regards their level of disease resistance. Despite the fact that the use of resistant cultivars cannot eliminate selectively the resistant strains, it can eliminate both resistant and sensitive isolates. Reducing the number of treatments with DMIs, by applying them only when environmental conditions are favorable for disease development, is a prerequisite for successful resistance management; therefore, the use of disease-resistant varieties could aid toward management of DMIs resistance in C. betico~a. KEY WORDS: Cercospora leaf-spot; demethylation-inhibiting fungicides; futriafol; fungi- cide resistance; Cercospora beticola; sugar beet. INTRODUCTION Sugar beet leaf-spot caused by Cercospora beticola is the most important foliar disease of sugar beet in warm and humid regions, including some areas in the Mediterranean Basin (20), and results in significant yield losses in the absence of control measures (2,23). Control of the disease is achieved mainly by fungicide spray applications. Sterol demethylation-inhibiting fungicides (DMIs) constitute one of the most important groups of fungicides playing a role in control strategies of the disease (1,2). In Greece DMIs have been used since 1979 for controlling sugar beet leaf-spot, always in mixture with a protectant fungicide, either maneb or chlorothalonil (8). During the last decade the DMIs used have been almost exclusively the triazole fungicides flutriafol and difenoconazole Received Aug. 15, 2002; received in final form Feb. 20, 2003; http://www.phytoparasitica.org posting May 6, 2003. 1plant PathologyLaboratory,AristotelianUniversityof Thessaloniki, Facultyof Agriculture, 54006Thessaloniki, Greece. * Corresponding author [Fax : +30-31-998854;e-mail: gkarao@agro.auth.gr]. 2Plant Protection Dept., Hellenic Sugar Industry S.A., Plati Imathias 59032, Greece. Phytoparasitica 31:3, 2003 275