One of my many knockout roses is getting leggy and losing yellow, discolored leaves. It's been really hot here but the others are doing well. Is it not enough water or a pest?
Why Is My Knockout Rose Dropping Leaves That Are Yellow?
Roses need regular deep watering, even the Knockout's which are tough shrub types. What appears to be happening is the roses are getting stressed from the high temps and need more water. You may also be dealing with insect problems, since the plants become more susceptible to insect damage once their stressed. Hopefully this article can help: