Question
I have 3 rose beds of dark pink knock out roses.  i pruned them in january (zone 8B)and they bloomed beautifully 2 weeks ago.  Since then, they have all practically defoliated.  i spray them regularly for black spot, etc.  i thought originally it was black spot because some of the leaves do have spots, but mainly are just yellow.  the ones that are not yellow yet you can barely touch them and the 5 leaflet clsuter falls off.  is there some nutrient they are lacking?  i have rainbow knockouts plus about 40 other roses in different beds that are fine.  the only thing different about these is that they were supplied by the landscaper 2 years ago and the others were purchased elsewhere.  
please help!
thank you!

Answer
It depends where you garden, but rarely are soils deficient in magnesium. However you can if you like try 1 tablespoon of Epson Salts in 1 quart if water. Use about a cup on each rose. I get a lot of questions on Knock Out roses and I am beginning to think that there are some sprays that they re-act to. Many people have said that they use a certain spray especially for Black Spot and the rose leaves go yellow. just a thought but it might be one of the problems. If your other roses are doing well then you are doing the right thing for growing roses well. I would  perhaps question the landscaper as to where he got the roses because it sounds like they were in trouble before you even planted them. Another reason could be that something different was done or added to the new rose bed. Can you think of anything that you have done differently?
However for now I would stop spraying them, cut them back if the top growth is poor so that it will activate them into normal healthy growth and if possible the Department of Agriculture will do a test on your soil for you. The other alternative is to purchase a cheap soil tester kit, a cheap one is fine, and see what comes up for you on the test. It is clear to me that the problem is either poor rose plants or the spray.