Knockout Rose Disease
Attached is a picture of the bottom of my knockout rose petal. I don't know what is wrong with my plant. It has good draining, gets full sun and I have fed with rose food. This is a container rose.
If you notice some webbing within the foliage like spiders do, it could be spider mites. It could be other small insects as well. I recommend getting some Bayer Tree and Shrub granules and apply it as directed on the label for container grown roses, ornamental shrubs. If only the lowest leaves are effected, it is easier for soils dwelling insects to attack them even in pots/containers.