Sometimes confused with Crown Gaul however this problem exists in the open air and not in the ground.
Caused by roses which want to send out roots from nodes. A callus forms at the nodes and turns into woody material when in the air.
Not a common problem but have seen on several varieties which are very easy to grow as cuttings. Their ability to form own roots makes them more susceptible to this problem. Seen widely on weepers Dorothy Perkins, Sea Foam and Bloomfield Abundance as well as the bush forms of these varieties, and Alba Meidiland.
Nothing can really be done to correct this problem however a strong prune will give new growth and the problem will have been removed for a time period.