Answer: This particular rose is very disease resistant so I am a little surprised you have had this problem. Probably the most important thing you can do now is clean up thoroughly this fall and winter. Clean up all the fallen foliage, during the winter pick off any remaining leaves, in spring when you prune them back remove all the prunings and put those in the trash. I would also replace the mulch beneath them to try to limit reinfection. I hope they are planted in a site with full sun and good air circulation and not too crowded together. Here is some general informationn on black spot that you may find helpful.
Unfortunately, many rose varieties have little or no resistance to black spot. (Not true for Knockout.) Innate susceptibility plus the weather as well as cultural conditions can all contribute to it appearing and to its severity in a given year.
This is a fungal infection. Damp conditions would of course encourage it to develop. Once black spot appears for the season, it is difficult to control. In general, remove affected leaves as soon as they appear and then replace the mulch immediately beneath the plant to try to keep the disease spores from splashing back up onto it. Keep the plant lightly thinned to promote air circulation and make sure it is planted in a location with good air circulation and in enough sun. If you water, avoid watering the foliage and water in the morning rather than at night. Finally, each winter, remove all of the leaves from the plant and dispose of them. Then replace the mulch beneath the plants. Also remove any prunings from the area. These steps should help limit reinfection.
Once you see signs of it developing, you can pick off the affected leaves, and consider spraying. People have reported success using products containing neem oil or containing sulfur (read and follow the label instructions carefully), or using a home-made spray containing one part milk and two to three parts water. Although some people also report success using a baking soda based spray, it can burn the foliage on some sensitive roses and according to the American Rose Society is better as a preventive than a treatment. It is usually a good idea to test spray a few leaves, wait a few days, and make sure there is no adverse reaction prior to spraying the entire plant -- some roses are extremely sensitive to sprays of any kind.
If your plants are in pots, you may also want to spray the pots and surrounding patio or deck to try to keep the spores from splashing back up onto the plants. (Test and make sure the spray will not damage these surfaces.)
Unfortunately, if you have a plant that is particularly susceptible to black spot you may never get rid of it permanently without a regular spray routine plus some cooperation from the weather and maybe some extra good luck. (With Knockout I think you have a good chance of controlling it. Good luck!)
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