09 My rose has black spots on the leaves, what do I do?
Description
This article is from the Rose Gardening FAQ, by Bill Chandler chandler@austin.ibm.com, Jolene Adams jolene@cchem.berkeley.edu, Brent C. Dickerson odinthor@csulf.edu, Karen Baldwin kbaldwin@veribest.com, and many contributors
09 My rose has black spots on the leaves, what do I do?
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
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, 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.
Preventitive treatments for blackspot
1. The least toxic spray for blackspot is baking soda. Combine 1
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.
2. Another treatment is sulfur dust used every 7-10 days.
3. Finally, there are the more toxic chemical fungicides. They are often
used once every week or two and are very effective. Follow the
directions carefully. Be especially careful to spray properly in hot
weather to avoid leaf burn. Spray the undersides of the leaves first.
4. 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.
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