could some of you experts explain dieback to me, i.e., what causes it, can the bush be cured, etc. need help, please.
Die back often result from rose stem canker. U of Illinois Extension has a good article (see below). A few points: 1) use 70% alcohol to sterilize cutting shears 2) cut to 1/4" above a bud, but NOT longer 3) use lime-sulfur spray before putting in winter-protection 4) rose dieback is often caused by WETness, either from poor drainage, or the stuff that piled on top (wet leaves).
I get cane dieback from POOR-Drainage indoor plants that sit on a saucer (indoor herbs like basil). I learn to use better drainage potting soil, rather than MiracleGro with too much peat moss and water retention. I get rose dieback from piling wet leaves for winter protection, or putting wet soil in a bucket (bottom cut off) to winter-protect. The roses that are green to tip are protected the dry-way, like piling up bark mulch to 12". David Austin Catalog recommends putting evergreen branches on top to winter-protect. Evergreen needles are very dry and good-insulation.
On winter-protection, U of Illinois stated that roses dieback more from rose cane canker (wetness around the stem). Keeping the root moist, but the stems dry is the key to prevent rose dieback from cane canker.
Here is a link that might be useful: U of Illinois Extension on Rose Cane Canker
"Dieback" isn't a particular pathology. There are several different fungi that cause stem canker, as the link explains. Verticilium wilt disease, botrytis blight, bacterial blight, winter damage, and cane borers (insect larvae) are other possible causes.
Canker disease is most likely to be active in the cool, moist conditions of spring, and it preferentially attacks younger canes that are still green. Some varieties are particularly susceptible. If it is canker, you see defined areas of dead bark, usually with concentric color zoning. It doesn't cause the cane to die back unless it girdles or nearly girdles the cane.
If you can post sharp, closeup pictures of your sick canes we might be able to give more help.
There also may be a genetic predisposition. Some cultivars consistently die back from the pruning cut ('Color Magic'). Are they highly susceptible to infection once wounded, or is die back a triggered response to wounding? That I would like to know.
Yeah hoovb, I'd like to know that too! Here we find it is very common in Belinda's Dream, even without obvious signs of canker. In a good, vigorous specimen (which most are) it is not a problem.
This year I pruned to open up the center of the plant more to see if that helps. I don't prune them as HTs, more like shrubs FWIW.