Blackspot is a fungus that causes black spots about 1/16 to 1/2 inches in diameter to form on the leaves and sometimes stems. The infected leaves later turn yellow around the spots and eventually fall from the plant. In bad cases, blackspot can severely defoliate a rose bush. The conditions that promote blackspot are wet leaves, splashing water and warm temperatures.

Here are some ways to combat blackspot. Most of these methods also apply to preventing and treating powdery mildew.

  1. Pick a variety of rose resistant to blackspot. For example, many Rugosas are quite resistant to blackspot.
  2. Use watering methods that don’t get the leaves wet: drip watering, using a soaker hose, or just soaking the ground with a light stream from a garden hose. If overhead watering is used, do so in the morning so the leaves can dry off before evening.
  3. Remove ALL diseased leaves from the plant or ground immediately to prevent further spreading of the disease. Infected leaves never get better, they just spread the disease. Prune infected canes severely in late winter.
  4. Prune away crossing canes and open the center of the bush to allow sunlight and airflow to more of the plant.
  5. Blackspot is transmitted by water splash. Remove leaves close to the ground (the first 6‑8 inches) which are more susceptible to getting water splashed on them. Mulch well to minimize water splashing onto leaves. If a plant had a lot of blackspot the previous year, remove the old mulch in early Spring, allow the area to dry and replace with clean new mulch.
  6. Keep the plant well watered. A weak or stressed plant is more susceptible to disease.

Preventative spray treatments for blackspot

  1. Chemical fungicides can be very effective in preventing blackspot and are usually applied every 7‑14 days. It is most important to spray the undersides of the leaves. FOLLOW THE LABEL DIRECTIONS EXACTLY. Too much fungicide can cause leaf burn. It is best if rose plants are watered well before spraying. Spraying during very hot weather can damage leaves. Early morning and early evening are the best times to spray. Avoid spraying under windy conditions. READ THE PRODUCT LABEL carefully and wear proper equipment when spraying, such as eye, mouth and nose protection.
  2. Since a single fungicide may not completely wipe out all the fungi, using that fungicide over and over may actually cause fungus to build up a resistance to that fungicide. Alternating between two fungicides, such as Triforine (Funginex) and Daconil, is recommended to keep resistant fungi from building up. Fungicides generally can prevent blackspot, but do not cure an existing case of blackspot.
  3. Some gardeners wishing to avoid fungicide use have tried using baking soda to help prevent blackspot with mixed results. Combine 1‑1/2 tablespoon baking soda and either 2 tablespoons horticultural oil or a few drops of Ivory liquid with 1 gallon of water. Mix as well as possible, and spray both sides of the leaves once a week. The Ivory liquid helps the baking soda stick to the leaves. Reapply after a rain. Baking soda changes the P.H. of the leaves, helping to prevent blackspot. Spraying with baking soda works for some gardeners, but others have found that baking soda is not effective enough in their climate.