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Originally posted by hami31

Xin các Bác hoặc các bạn nào biết chỉ dùm cách trị những cây Hoa hồng hay bị vàng lá và có nhiều chấm đen.

Xin chân thành cám ơn trước.

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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 plashing 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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