In the lower reaches of the Yangtze River, the climate is generally mild with moderate temperature and humidity, along with sufficient sunlight. These conditions create an ideal environment for the growth and spread of certain pathogens, often leading to severe damage in ornamental plants. To help gardeners and horticulturists manage these issues effectively, this article outlines the symptoms and control methods for three common plant diseases.
**1. Powdery Mildew**
Powdery mildew primarily affects the leaves, but it can also infect petioles, stems, buds, and even petals. Initially, small white powdery spots appear on the infected areas, which gradually expand and spread. Under favorable conditions, a full leaf can be covered within 7–10 days, giving the appearance of frost. The affected leaves may become distorted, wilted, or brittle, significantly impacting the plant’s growth and photosynthesis. Some plants may fail to bloom properly, while others may die. Commonly affected plants include chrysanthemums, impatiens, euonymus, cineraria, dahlias, roses, and many others.
To control powdery mildew, ensure good air circulation by placing pots in well-ventilated areas. Avoid excessive nitrogen fertilizer and overwatering. Remove infected leaves and cut back diseased branches promptly. At the early stage, a 1:1:140 Bordeaux mixture can be applied. Once symptoms appear, use 25% triadimefon at 1000 times dilution or 70% thiophanate-methyl at 1000 times. Repeat the application every 10 days for 2–3 times for best results.
**2. Brown Spot Disease**
Brown spot disease mainly affects the leaves, starting as small brown spots that gradually enlarge into irregular lesions. A pale halo often surrounds the spots, with reddish-brown edges and a gray-white center. In later stages, black spores (conidia) may form. Severe infections can cause leaves to fall off, leading to bare stems and eventual plant death. This disease commonly affects rhododendrons, jasmine, camellias, water lilies, and many other flowering plants.
To manage brown spot, remove and burn infected leaves immediately. Before the disease occurs, spray a 1:1:200 Bordeaux mixture every 7–10 days as a preventive measure. During early infection, apply 65% zoxin WP at 500 times or 1000 times diluted thiophanate-methyl. Alternatively, use 50% carbendazim at 200 times for irrigation.
**3. Anthracnose**
Anthracnose typically appears on leaves and can also affect stems, petioles, and buds. Infected leaves show small, water-soaked spots that turn gray-brown with brown edges and slight elevations. The center eventually fades to white, and the lesions become thin and translucent, sometimes leading to holes. In wet conditions, red mucilage may appear on the lesions, indicating spore production. Affected plants may lose their leaves and eventually die. Commonly affected plants include camellias, roses, orchids, lilies, and many others.
Control measures include proper watering, avoiding over-fertilization, and ensuring good air circulation. Remove and destroy infected leaves and stems immediately. At the onset of the disease, apply 50% anthrax omeprazole WP at 500 times, 70% thiophanate-methyl at 1000 times, or 75% chlorothalonil at 800 times. Repeat the treatment every 10–15 days for 2–3 times, alternating between different fungicides for better effectiveness.
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