Pathotype-specific powdery mildew resistance in roses

Differences in resistance to two powdery mildew isolates (R-P and R-E) were evaluated on two rose genotypes, Rosa wichurana and ‘Yesterday’. Pathotype-specific resistance reactions to the developing fungus were observed by microscopy. Both rose genotypes expressed a comparable reaction, i.e., cell reactions to mycelium development of R-P. On ‘Yesterday’, pathotype R-E stopped growing after germination, whereas R. wichurana showed cell reactions upon inoculation with R-E. Crosses between the genotypes ‘Yesterday’ and R. wichurana resulted in some offspring with transgressive segregation for powdery mildew resistance. Most of the F1 hybrids showed partial resistance by cell reactions, while others segregated for the arrested development of the germ tube of R-E as seen in ‘Yesterday’. These reactions suggest that pathotype-specific resistance, based on different resistance mechanisms, can be used for resistance pyramiding in rose breeding practice.