Characterisation of powdery mildew resistance in a segregating diploid rose population.

Powdery mildew (Podosphaera pannoso) is one of the most serious fungal diseases on both greenhouse and field grown roses. Improvement of disease resistance is a major selection aim for garden rose breeders. For rose cultivars, being mostly tetraptoid, it is complicated to develop molecular markers for resistance. Hence, a segregating diploid population was established from a cross between 'Yesterday', a commercial available rose variety susceptible to powdery mildew, and R. wichurana, a rose species with resistance to certain isolates of powdery mildew. A progeny of 94 seedlings was planted in the field. The segregation of powdery mildew resistance was studied in this population by means of a bioassay with two different monoconidial isolates of powdery mildew. Based on the response to these inoculations different groups were selected: a first group of genotypes was susceptible to both isolates, other groups were susceptible to one of both isolates and a last group was resistant to both tested isolates. The disease resistance inherits for both isolates in a quantitative way. A genetic map based on AFLP and SSR markers was established and will be used for QTL analysis of powdery mildew resistance.