Fungicide sensitivity of South African apple scab populations

Abstract: The most effective control of apple scab (Venturia inaequalis) is still achieved throughthe use of fungicides. However, monitoring of efficacy of active ingredients is required to avoidunnecessary cost, environmental pollution and health exposure. In this study, the current status ofefficacy of two commonly used classes of fungicides, namely one anilinopyrimidine and anergosterol biosynthesis inhibitor, were evaluated. Sampling was conducted in two orchards in theWestern Cape Province of South Africa. In planta fungicide sensitivity was similar for bothpopulations, with a shift towards loss of fungicide sensitivity with rare practical cyprodinilresistance occurring. The average ED50 of one population (sampled year 2006) was estimated tobe at 23μg/ml for cyprodinil and 14.5μg/ml for flusilazole, whereas for the second population(2010), the ED50 was 24μg/ml for cyprodinil and 19μg/ml for flusilazole. Single-spored isolateswere pre-screened in vitro using a discriminatory value of 0.1μg/ml or 1μg/ml of flusilazole and0.1μg/ml or 3μg/ml of cyprodinil to compare the relative growth of sensitive vs. resistant isolates.On average, isolates with shifted flusilazole sensitivity showed an ED50 of 0.036μg/ml. Only 23%of isolates with a shift in anilinopyrimidine sensitivity were also found to have reduced EBIsensitivity in vitro. In addition, 20 isolates per orchard were single-spored and tested for mutationin the ABC transporter gene through PCR-RFLP and sequencing. Polymorphisms potentiallyaffecting the function of the encoded protein were found in the nuclear binding region of theABC 2 transporter.