When black spot disease becomes severe on roses, it can cause the lower and middle leaves of the entire plant to fall off, 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 purple-brown round spots or irregular lesions, which may not be immediately noticeable. As the disease progresses, these spots become more distinct, turning into dark purple or brown patches with visible radiating patterns. In advanced cases, the lower leaves may appear pale, and the branches might dry out.
The pathogen overwinters in dead leaves or soil, and conditions such as poor air circulation, lack of sunlight, waterlogging, poor hygiene, and improper watering or fertilization practices can promote disease development. Prevention is key in managing black spot. Early detection and treatment are essential, as once the disease spreads, it becomes much harder to control using pesticides.
When planting roses, choose disease-resistant varieties and maintain proper care for seedlings. Water the roots rather than the foliage, and if possible, use drip irrigation, furrow irrigation, or basin watering. Avoid watering at night to prevent moisture from lingering on the leaves, which can encourage bacterial infection. Prune the plants to improve air circulation and ensure at least 6 hours of daily sunlight. During the growing season, reduce nitrogen fertilizer and increase phosphorus and potassium to strengthen the plant’s resistance.
Regularly remove infected leaves and keep the area clean to minimize the spread of the disease. In winter, heavily prune affected stems to eliminate overwintering pathogens. Avoid touching diseased leaves to prevent accidental transmission.
In early spring, before the rose buds begin to open, spray a 100-times dilution of lime sulfur solution. During summer, when new leaves start to unfurl, begin spraying regularly until winter. At the first sign of disease, alternate between 75% chlorothalonil wettable powder diluted 1000 times, 45% thiabendazole (Tecto) suspension diluted 500–600 times, 40% flusilazole, or 25% nitrile emulsifiable concentrate diluted 800 times. Spray daily during rainy periods, and every 7–10 days during normal growing seasons. Apply 4–5 treatments to effectively control the disease. If the infection is severe, you can use 70% mancozeb, 30% copper oxychloride suspension diluted 800 times, 75% chlorothalonil at 500 times, or 12.5% triadimefon emulsifiable concentrate diluted 1000 times. Spray every 7–10 days, up to 3–5 times. Among these, 12.5% triadimefon (Spotted Off) at 1000 times and 75% chlorothalonil at 500 times show the best results.
For more detailed information, visit the **China Pesticide Network**.
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