My Knockout Roses developed blackspot, apparently last summer, probably because my husband uses the lawn sprinkler all the time….(I know, I know). I have cut them down, leaving about 4-6″ of each bush, but the remaining canes do have blackspot. However, I do see new growth (leaves) coming out of the spotted canes, and I’ve already sprayed them once. Will they survive? Should I continue spraying them every so often?
It would be best if you can cut off the canes with the black spots. Roses are so susceptible to insects and diseases that a regular spraying program should be followed whether you see a problem or not. I’d start out using Bonide Rose Rx every couple of weeks.. The neem oil is a good insect control as well as a decent preventative fungicide. If problems show up anyway, then something stronger may be in order. If you can’t stop your husband from spraying them with water regularly then I think you can count on a worsening situation.