Orchid anthracnose is a prevalent and serious disease affecting orchid cultivation in South China. It is commonly found across the region and can cause significant damage to orchids, as well as other plants such as poinsettia, jasmine, privet, bergamot, and hibiscus.
Symptoms of the disease typically begin with small reddish-brown spots on the leaves, which gradually expand into larger gray, grayish-white lesions. The affected areas on the leaves may appear semi-circular, round, elliptical, rhombic, or irregular in shape, ranging from 0.5 to 1.2 cm in length and 0.3 to 0.5 cm in width. These lesions may spread into large patches or cause the leaf tips to wither. The centers of the lesions are often dark brown or gray, and some may be surrounded by a yellow-green halo. In advanced stages, small black specks (spores of the pathogen) appear on the surface of the lesions, which can break open easily, leading to yellowing of the leaves and a decline in ornamental value.
The causative agent of the disease is a fungus belonging to the genus *Mycoplasma*, though it is not fully identified. The pathogen survives in infected plant debris and soil during winter. In spring, it produces conidiospores that enter the plant through stomata or wounds, especially after rain or insect activity. The optimal conditions for spore germination are temperatures between 20–25°C and relative humidity above 80%. In Guangzhou, the disease is more active during early spring when conditions are favorable, particularly in May and June when high humidity and continuous rain create ideal environments. It can also occur in autumn, especially in September and October. Poor management practices—such as overwatering, overcrowded planting, leaf damage, and poor drainage—can worsen the disease. In northern greenhouses or home settings, orchids are also susceptible year-round due to factors like dense planting, excessive watering, and poorly drained soil.
To manage orchid anthracnose effectively, several strategies should be implemented:
1. **Improve cultural practices**: Avoid over-crowding, protect plants from frost, and handle them carefully to prevent injury. Use non-porous pots and water appropriately, avoiding direct pouring. Ensure good air circulation and light exposure. Apply balanced fertilizers, focusing on phosphorus and potassium to boost the plant’s resistance.
2. **Remove infected material**: During winter and spring, promptly remove and destroy diseased leaves and stubs to reduce the infection source. Apply a 0.5% to 1% Bordeaux mixture to the soil and plants, repeating this 1–2 times.
3. **Chemical control**: At the early stage of infection, spray 50% Mancozeb WP at 500 times dilution or 30% Propiconazole WP at 2000 times, applying every 10 days for 3–4 treatments. During the outbreak, use 50% Thiophanate-methyl WP at 800 times, 50% Mixed Sulfur Suspension at 700 times, 25% Carbendazim at 1000–1500 times, 50% Benomyl WP at 1000 times, or a mixture of 69% Chlorothalonil + 75% Mancozeb (1:1) at 1000–1500 times. Other options include 65% Triadimefon WP at 800 times, 75% Chlorothalonil + 70% Thiophanate (1:1) at 800–1000 times, or 25% Carbendazim at 500 times. Spray every 7–10 days, alternating products for 3–4 applications to achieve effective control.
4. **Protect new growth**: Regularly apply protective sprays to young leaves. Options include 1% Bordeaux mixture, 75% Chlorothalonil + 70% Thiophanate (1:1) at 800 times, 80% Mancozeb at 600 times, 50% Benzimidazole at 800 times, or 20% Tricyclazole at 800 times. Spray every 7–10 days for 3–4 consecutive times to maintain protection and control the disease effectively.
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