Yuck! I don't know what could have caused it. I have had great luck with most of my Knockouts with very little attention. I am struggling with 2 of the Pink Knockouts. They were eaten pretty severely by deer 2 winters ago and are having trouble bouncing back. This Spring I noticed one of the canes looked diseased. I cut it back and I'm hoping for the best. I don't want to dig it out because I interplanted it with some Clematis integrifolias and I don't want to damage the rootball of the clematis in the process. I hope you find out the cause of your problem and can get it under control. I personally love my Knockouts for their constant color and disease resistance, although I know many here who loathe them because they are so common. This is the same way I feel about Stella d'Oro daylilies so I understand the sentiment but I still like my Knockouts and I'm considering getting the Yellow one.
Good luck!

ROSE SLUGS.
(Sawfly larvae)
They are on the UNDERSIDE of the leaf.

Looks like a pretty good colony of them.

Jeri

I usually just squish them. Although you could try sprinkling them with flour as they always seem moist looking - they might suffocate with the flour.

Carol

I usually just squish them. Although you could try sprinkling them with flour as they always seem moist looking - they might suffocate with the flour.

Carol

Thanks, guys. There's a certain satisfaction in squishing. I'll give it a try.

Safer organic insect spray. I squished a lot of disgusting sawfly larvae, but there are just too many of them, and I was FED UP!