Lack of influence of host plant disease resistance on the evolution of resistance to sterol demethylation-inhibiting (DMI) fungicides inCercospora beticola

Field experiments were conducted during 1997 and 1998 to determine the effects of sugar beet cultivar susceptibility to Cercospora leaf-spot on the sensitivity ofCercospora 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, reflecting 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 inC. beticola.