If you happen to notice small circular black spots, your plant probably has what is known as the Black Spot. The spots have fringed edges, and are a sign of a disease. Artificial sprays may be used to treat the spots, but the affected leaves should be removed from the plant, as remaining leaves will allow the disease to spread to the rest of the plant.
If you notice that the leaves on your rose plants start to curl and turn purple, you should check the leaves and canes for a white powder, as these may be signs of a fungal infection known as powdery mildew. Spread by the wind, it may cause the canes to become stunted or malformed. Treat using Funginex or Benomyl.
Known as rust, this disease is characterized by orange-red blisters that turn black in fall. It can survive the winter and will then attack new sprouts in the spring. Collect and discard leaves that are infected in fall. A Benomyl or Funginex spray every 7-10 days may help.
If you start to see yellow, red, or green spiders on the undersides of the leaves on your roses, you have spider-mites, which will suck the juices form the leaves the longer they are there. they are easily detected by spotting the leaves and flowers that are stunted or malformed. You can use Isotox or Orthene to aid you in treating this infestation.
If you see small red, green, or brown bugs under your plants, you have an infestation of aphids. Tell-tale signs of an infestation are mottled leaves and small white webs underneath the leaves. Oft-times the bugs will be found clustered under leaves or near the tender buds, where they will suck the juices. You can use Diazinon or Malathion to clear the infestation.
If your flowers don’t open, or are deformed when they open, Thrips could be the reason behind the problem. They are slender, brown-yellow bugs with fringed wings that also suck juices from flower buds. Cut and discard the infested flowers. Orthene and malathion may also treat this problem.