Hi Laura, that looks like a spider mite infestation to me. I'd remove all the foliage within six inches of the ground then remove all foliage from the inside of the bush. Using a water wand with a high pressure spray of water, wash the bush thoroughly every day for three or four days, concentrating the water spray on the undersides of the leaves where mites hide.

Get a 6x hand lens and look at the underside of lower leaves that aren't dead yet. If you have a spider mite problem, you will see several mites on every leaflet along with yellow eggs and webbing. The lowest leaves develop a grayish pallor on the top side with fine pale (not green) stippling. Infested leaves become progressively brittle and slowly die. This does not look like mite damage to me. But if you develop a mite problem, Cecily's advice is perfect.

The brown V-shaped spots are a probable fungus that affects roses resistant to black spot, and I have seen it on Knock Out. No control is needed.

Solid yellow leaves are just dead leaves that the rose has decided to discontinue. Mottled green and yellow leaves are in transition to solid yellow. We can see that some of these leaves have old insect or fungal damage or are in deep shade.

Although I've never noticed this many yellow leaves on a Knock Out, I wouldn't be alarmed by what I see in these photos. I would remove all the yellow and brown leaves and see what happens. The younger growth looks good.

Thanks for your input. I looked at the underside of the leaves, but I don't have a magnifying glass and can't see anything with the naked eye. Gotta get a magnifyer. I did, however, find small clumps of dead brown leaves toward the bottom of the plant that looked like they had spiderwebs on them. I cleaned up the bush pretty well, pulling all the brown and yellow leaves off. I'm grateful that the other 3 roses in close proximity look fine. I find it strange that the same thing happened to it last year. I had thought I would need to replace it, but it looked fine in the spring.