Monitoring of Venturia inaequalis strains sensitive to strobilurin fungicides and occurrence of apple scab on resistant cultivars in the Czech Republic

Abstract: Occurrence of apple scab (Venturia inaequalis) on resistant cultivars was investigated in the Czech Republic however symptoms have never been observed up to the year 2006. Apple scab is currently recorded in six isolated plantings of resistant cultivars in the territory of the Czech Republic. Apple scab was founded only on Vf resistant cultivars (Rubinola, Topaz, Rajka, Otava, Melodie etc.) in all cases indicating that those isolates can be classified as the race 6 or 7. Monosporic isolates of V. inaequalis were prepared for next testing using plant indicators and distinction using PCR methods.At the same time, sensitivity of V. inaequalis to strobilurine fungicides was tested in the orchards, where the chemical treatment against apple scab was ineffective. Leaf samples were collected from 22 commercial orchards, one sample was taken from apple tree solitary growing in natural conditions and one sample was taken in experimental orchard. A germination of spores in aqueous fungicide solutions was assessed. A decrease of strobilurine sensitivity of V. inaequalis was observed in several localities.