When the black spot disease of roses becomes severe, it can lead to the complete loss of lower and middle leaves across the entire plant, leaving only a few new leaves at the top. This significantly impacts the plant’s growth, flowering, and overall ornamental value. After infection, the leaves develop various purple-brown circular or irregular-shaped spots that may be subtle at first. As the disease progresses, the spots become more distinct, appearing as dark purple or brown lesions with clear radiations. In extreme cases, the lower leaves turn yellow, and the branches begin to dry out.
The bacteria responsible for black spot overwinter in dead leaves or soil. Poor air circulation, insufficient light, waterlogging, unhygienic conditions, and improper watering or fertilization practices all contribute to the development of the disease. Prevention is key—early detection and treatment are far more effective than trying to control the disease once it has spread. Once the infection becomes widespread, managing it through pesticide use alone becomes very challenging.
When planting roses, it's best to choose disease-resistant varieties and maintain proper care for the seedlings. Watering should be done at the root level rather than spraying the foliage. If possible, drip irrigation, furrow irrigation, or basin watering is recommended. Avoid watering at night, as this can leave moisture on the leaves, creating an ideal environment for bacterial growth. Prune the plants regularly to improve air circulation and ensure they receive at least six hours of sunlight daily. During the disease outbreak, reduce nitrogen fertilizer and increase phosphorus and potassium to strengthen the plant's resistance.
Keep the leaves clean by removing infected ones promptly, which helps reduce the spread of the pathogen. In winter, heavily prune the plants to eliminate overwintering pathogens on diseased stems. Avoid touching the leaves of infected plants to prevent human transmission of the disease.
In early spring, before the rose buds start to open, apply a 100-fold dilution of lime sulfur solution. In summer, when new leaves begin to unfurl, start spraying regularly until winter. At the first sign of infection, alternate between 75% chlorothalonil wettable powder (1000 times dilution), 45% thiabendazole (Tecto) suspension (500–600 times), 40% flusilazole, or 25% fenoxycarb emulsifiable concentrate (800 times). During rainy periods, spray daily; otherwise, spray every 7–10 days. Apply the treatment 4–5 times to effectively control the disease. If the infection is severe, you can use 70% mancozeb, 30% copper oxychloride suspension (800 times), 75% chlorothalonil (500 times), or 12.5% triadimefon emulsifiable concentrate (1000 times). Spray every 7–10 days, up to 3–5 times. Among these, 12.5% triadimefon (1000 times) and 75% chlorothalonil (500 times) have shown the most effective results.
For more detailed information, visit the **China Pesticide Network**.
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