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. Many Rugosas are quite
resistant to blackspot. Some yellow Hybrid Teas are especially prone to
blackspot. However, yellow Floribundas such as Sunsprite seem to be
especially 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
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, replace
the old mulch with clean new mulch in Winter or early Spring.
6. Keep the plant well watered. A weak or stressed plant is more
susceptible to disease.
For more information go to the source i recomend. I hope that helps you out!
· 10 years ago