Q.white patches on rose stem
There are white patches forming on stem and after some days the stem turns to black color. What should I do now?

That photo sure looks like a bad case of Scale attacking the rosebushes. I would recommend using either the product at this link: http://www.planetnatural.com/product/take-down-garden-spray/ or get some Bayer Rose and Flower Care granules and give the rosebushes some of the granules mixed into the soils ASAP. The granules are a systemic that will move through all the tissues of the rosebushes and will provide a longer lasting control.