A quantitative real-time polymerase chain reaction (qPCR) was developed and validated for quantification of Venturia inaequalis in infected leaf tissue of Malus × domestica. The method is based on dual-labeled hybridization probes, allowing simultaneous detection of host and pathogen DNA within one single reaction. Limit of quantification for the pathogen was 0.5 pg per reaction and, for the host, reached 5 pg per reaction. The fungal growth measured in four apple cultivars 2 weeks after inoculation significantly correlated with their different level of scab resistance and allowed the observation of ontogenic resistance. After sporulation on the youngest leaf, fungal biomass in susceptible 'Gala' was 118 times higher than in resistant 'Florina' and 'Discovery' while intermediate values were found with the intermediate susceptible 'Milwa'. Correlation was also observed between severity classes obtained by visual scoring of symptoms and qPCR results. Moreover, qPCR demonstrated validity of the developed method as a disease severity forecast tool 10 days after the pathogen's inoculation and prior to the appearance of the symptoms. Applications of the methodology can include the quantification of scab resistance during breeding programs, evaluation of fungicide and biocontrol efficacy, and quantification of the fitness of different pathogenic strains.