Powdery mildew resistance in roses is pathotype specific

Powdery mildew in roses causes economic losses and is problematic in pot, cut and garden roses. Pathotypes of rose powdery mildew are well described and various resistance mechanisms are observed in different rose genotypes. In present study, Rosa wichurana and Rosa ‘Yesterday’ were found to react differently towards two powdery mildew pathotypes (R-E and R-P) by pathotype specific responses in the rose leaves. On ‘Yesterday’ conidia of R-E could germinate but no further development of mycelium was observed. On R. wichurana pathotype R-E developed but cellular reactions in the leaves were abundant. Both rose genotypes showed susceptibility to R-P. Fungal development of R-P was observed along with cellular reactions in the rose leaves. For all rose-pathotype combinations responses of the host plant were described and quantified. An F1 population (90 genotypes) obtained from ‘Yesterday’ and R. wichurana was used to develop a genetic map. Fungal development in the F1 population was scored for the pathotypes R-E and R-P. For both pathotypes the scores segregated differently resulting in the development of pathotype specific Quantitative Trait Loci (QTL) on the genetic map. Our results indicate that resistance reactions to powdery mildew do not only depend on the different resistance mechanisms present in roses but also on the specific pathotype. These results show that breeders who aim to select for better resistant roses need to take into account the interaction between plant and fungus. The best strategy to obtain roses with a durable resistance is to combine different resistance genes and subsequent resistance mechanisms thereby increasing the number of pathotypes withstood.