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Rose foliage infected with black spot.4. Clip off any diseased foliage. Fungal diseases, such as black spot and powdery mildew, can overwinter and reinfect plants the next spring. 5. Rake away fallen leaves and other rose debris, which can also harbor harmful fungal diseases and insects. Don’t compost any leaves, as the fungal diseases will still survive.
Disease. Roses can be affected by fungal diseases, such as blackspot and powdery mildew. Preventing fungal disease as well as recognizing the signs early are fundamental in caring for your roses. These diseases thrive in moist conditions and can largely be prevented or minimized by planting rosebushes in sunny areas and far enough apart to ensure good air circulation. Watering at the base of the rose, instead of from up above, will help keep roses drier, helping to inhibit the growth of disease. Finally, cleaning up all rose debris off the ground will remove any fungus and prevent its spread.Blackspot: As its name suggests, this fungus begins as a small black or dark brown dot that becomes yellow on the foliage. It can also affect the stems. Severe infestations will cause significant leaf drop and can kill your rosebush. Blackspot thrives in humid climates. To control, begin by removing all affected leaves and branches and disposing of them. Then treat remaining leaves with a fungicide — neem oil is a good organic option, although there are chemical fungicides available.Powdery mildew: This reveals itself as powdery-looking spots primarily on the leaves of roses, but it can also be found on the petals and stems. Warm weather and cool nights can create ideal conditions for this fungal infection to flourish. To treat, remove all diseased parts of the plant and treat healthy foliage with a fungicide such as neem oil, or spray with 1 tablespoon of baking soda mixed with one-half teaspoon of dish soap in a gallon of water.