My knockouts were hit with black mold at the end of summer last year. Before the winter I tried a solution of milk and water but it didn't help. I also tried a fungal spray from the garden center with no help. As you can see from the photo, it is all over the entire plant. Now that spring is a few days away, what steps do you recommend? I'm worried if I try a new plant in its place, the new plant will just get diseased from the soil. I have 4 knockouts that look like this. It's out of control!!
Black Spot on Knockouts
It looks like your roses have canker attacks going on. Usually a happy and growing well rosebush will fight the canker off with just a little help. I usually pruned the rosebushes back and do a shaping pruning once the leaf buds start to open a bit. The ends of the rose canes are sealed with either White Elmer's All Purpose Glue or Tacky Glue from a hobby store. Then spray the rosebushes down well with Immunox three times 7 to 10 days apart. Then switch to another fungicide of your choice every 10 to 12 days for three additional sprayings. This will help the rosebush fight off the canker and any other fungal attacks that may be going on. As long as you still have alot of green cane and by the photo you do, the rose should be able to fight it off. In some cases the canker only lessens but hangs on a bit. Those areas can be pruned out as the bush grows more. I try to prune out the heavier concentrations so that the rosebush has better odds of winning the overall battle. Here is a link to an article I wrote on the canker for you: https://www.gardeningknowhow.com/ornamental/flowers/roses/rose-canker-fungus.htm