This disease is commonly known as black spot. Black spots appear as circular with fringed edges on leaves. They cause the leaves to yellow. Remove the infected foliage and pick up any fallen leaves around the rose. Artificial sprays may be used to prevent or treat this kind of rose disease.
2. Stunted or malformed young canes
Known as powdery mildew, this is a fungal disease that covers leaves, stems and buds with wind spread white powder. It makes the leaves curl and turn purple. Spray with Funginex or Benomyl to treat this fungal disease.
3. Blistered underside of leaves
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.
4. Malformed or stunted leaves and flowers
This is caused by spider mites. They are tiny yellow, red or green spiders found on the underside of leaves where they suck juices. The application of Orthene or Isotox may help in treating this infestation.
5. Weak and mottled leaves with tiny white webs under them
This is caused by aphids. They are small soft-bodied insects that usually brown, green or red. Often clustered under leaves and flower buds, they suck plant juices from tender buds. Malathion or diazinon spray may help roses to survive these bugs.
6. Flowers that don't open or are deformed when they open.
Thrips could be the reason behind this 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.
Remember that roses are hungry feeders that require much fertilizer to become healthy bushes.