I have three white Rosa Canina and the one on close inspections has brownish...
Unfortunately, without seeing a sample closeup or first hand it is difficult to identify if you may be seeing a scale insect or something else. You may want to submit more photos. A closeup of the stem plus a picture of the what they look like when removed (top side and underside).
You could also take samples into the Douglas County office where they could take closeup photos with the microscope. There address is:
1325 Waterloo Lane
Gardnerville, NV 89410