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It could be rabbits, deer, snails or slugs. Without a description, one would hazard a guess.

This might help: https://www.gardeningknowhow.com/ornamental/flowers/roses/rose-bush-diseases.htm

This link will take you to several articles about roses: http://www.google.com/cse?cx=012078378210706707791%3Af1h5n_k1r5e&ie=UTF-8&q=black+spot+on+roses&oq=black+spo&gs_l=partner.3.0.0l5j0i5l2.10972.15364.0.18278.9.9.0.0.0.0.913.3387.2j1j1j5-3j1.8.0.gsnos%2Cn%3D13..0.0.6425j8497011j10..1ac.1.ifany6VYYt8#gsc.tab=0&gsc.q=black%20spot%20on%20roses&gsc.page=1

This particular article might be the one to answer your question: https://www.gardeningknowhow.com/ornamental/flowers/roses/yellow-rose-leaves.htm

A few yellow leaves aren't too troublesome for the most part, but if 20% or more of the plant is involved, something should be done to correct the situation.

Pruning back your knockout rosebushes will usually stimulate more growth, just doing a 6 to 8 inch pruning back should be sufficient to achieve this stimulation. Yellow blooming roses usually do have a problem with turning white fairly quickly. There are a few that hold a nice yellow color but most of those are more of a deep golden or redish yellow. So that much is pretty common. Here are some articles on Knockout rosebushes that you may find interesting:

https://www.gardeningknowhow.com/ornamental/flowers/knockout-roses/prune-knockout-roses.htm

https://www.gardeningknowhow.com/ornamental/flowers/knockout-roses/care-knockout-roses.htm

https://www.gardeningknowhow.com/ornamental/flowers/roses/self-cleaning-roses.htm

I would dig out the clay soils at least 20 to 24 inches in diameter and 18 to 20 inches deep and replace with a good gardensoils mix such as some the EKO brand sells. Hopefully they have these bagged soils at Home Depot or a Lowes near you. Place about two inches of 3/4" gravel in the bottom of the planting holes, then cover with landscape fabric. Now fill the planting hole with the new bagged soils and the rosebush. Feed the rosebushes with a good organic based rose food, such as Gro-Rich Rose and Perennial Food. You can also get some Moo Poo Tea Bags and Alfalfa Tea bags from Annie Haven at Haven Products, make up some tea and give some to each rosebush, it gets them to growing nicely and builds the soils home. Adding some root stimulator and super thrive to their normal waterings will help as well. There are some roses developed by Dr. Griffith Buck called Buck Roses, they are very hardy and tough. I love one of his called Distant Drums. Do a search on line for Haven Products or Moo Poo Tea to find the tea bag products and a search for Buck roses too.

Yes you certainly can. Rosebushes perform at their very best with lots of sun but some roses perform quite nicely with only partial sunshine, one of those happens to be the Knockouts. For a red blooming rosebush that looks alot like the red Knockout, try to find a rose named Winnipeg Parks. It is very hardy and far less likely to have problems with the Rose Rosette Virus. Not all Knockouts have problems with Rose Rosette but more so than when they first came out.