Is the observed differential reaction of rose powdery mildew on Rosa 'Yesterday' correlated with pathotype groups?

In rose powdery mildew two pathotype groups can be discriminated based on the differential reaction in rose and Prunus avium. Isolates from both groups can be identified by a one basepair difference in their ITS sequence. In previous research, the pathotype only infectious on rose was called R-E, while R-P infects both rose and Prunus. A specific resistance response characterized by a complete stop of development of germinated conidia was found on Rosa ‘Yesterday’ for pathotype R-E, while R-P could develop on ‘Yesterday’ by growing mycelium and the formation of new conidia. In current research we investigated whether the observed resistance response in ‘Yesterday’ could be correlated with the different pathotype groups. Therefore seven new isolates were collected and monoconidial cultures were established on tissue cultured roses. The pathotype groups were identified based on the ITS sequence. Three isolates belonged to the R-P type, while the other four were R-E type. Detached leaves of ‘Yesterday’ were inoculated with all new isolates and the fungal development was evaluated after lactophenol blue staining. The development of the conidia was defined by three different responses: not germinated, germinated and stop of further development and germination followed by normal development. For all seven isolates between 40 and 56% of the conidia didn’t germinate after inoculation. Germination followed by a complete stop of further development was only seen in the isolates belonging to the R-E pathotype group, and not in the isolates belonging to the R-P pathotype group. However in the new isolates of the R-E type also normal conidia development was observed. These results indicate the specific response of conidia germinating, followed by a stop of development is specific for the pathotype group R-E although in most isolates this response is not absolute for all conidia.