What is this disease? I have only read about it in an unm Table 8 list. I think that one of my Martin Frobisher seedlings has something that is not black spot but is much more red and follows viens rather than being round. The interesting thing is that this “thing” does not seen to affect the general vigour. Does any one know breeding behaviour, which plants are more prone?
Johannes, do you have a photo of it?
Aren’t purple spots following veins a sign of downy mildew?
It sounded that way to me also but with Downy you will lose leaves very, very fast and lose the plant in short order if not treated.
There is no defoliation.
Going back to Dr. Barnes talk about rose diseases at the ARS/WFRS meeting in Houston, Dr. Barnes talked about when Downey Mildew hit rose plants in Texas and the symptoms were not identical to those in CA/AZ which were so well documented. He reported the rapid defoliation but with much smaller spots and an inability to keep the disease going in his research facility in College Station TX. (I was one of many who bought the tape of his talk.)
If you have access to a zoom microscope or a friend with one, have a search made of the underside of the leaf with spots. DM has the ‘species’ name sparsa because the fungal material on the underside of the leaf is sparse, as opposed to rust and classic black spot (Diplocarpon rosae). Just as there are multiple races of Black Spot and Powdery Mildew, there may be more than one race of DM.
Two other possibilites are Cercospora and Anthracnose, both of which retain their leaves.