What are the main diseases affecting camellia plants and how can they be prevented?
Anthrax
[Causes and Symptoms] Anthrax in camellia is caused by infection with the fungus Colletotrichum gloeosporoides. This is one of the most common diseases affecting camellias, with an incidence rate of around 33%. The disease typically appears on leaf margins, tips, and veins. Initially, dark green spots appear and gradually expand into irregular, large lesions. Over time, these lesions turn from brown to black. In severe cases, the entire leaf may become affected, leading to heavy leaf drop.
[Incidence Factors] The occurrence of anthrax is strongly influenced by temperature and humidity. The ideal temperature range for disease development is between 25°C and 28°C. High humidity, especially during prolonged rainfall, accelerates the spread of the pathogen. The disease usually begins in April and peaks between June and July. After September, it tends to subside.
[Control Methods]
1. Remove infected leaves promptly to prevent further spread.
2. Prune dead branches in winter to improve air circulation and reduce fungal buildup.
3. Apply a 1% Bordeaux mixture every two weeks before the start of each growing season.
4. Spray with a 600-fold solution of chlorothalonil once a week, repeating 3–4 times.
5. Improve cultivation practices by using proper weeding and fertilization. Apply phosphorus and potassium fertilizers in spring, and spray a 0.15% potassium dihydrogen phosphate solution before new leaves emerge.
Algae Disease
[Causes and Symptoms] Algae disease is caused by the parasitic rust algae Cephaleuros virescens. Lesions appear mainly on the upper surface of the leaves, starting as small gray-green dots that grow radially into nearly circular or irregular patches. These lesions become raised and covered with a fine, felt-like texture. As the disease progresses, the color changes to dark brown. This condition reduces the plant’s ability to photosynthesize, weakening its overall growth.
[Disease Cycle] The algae overwinter in infected tissues as filamentous structures and produce zoospores under wet conditions. These spores infect new plants, particularly when there is poor ventilation and low light. The disease is more severe in Camellia oleifera plants that are weak due to inadequate sunlight or poor air circulation.
[Control Methods]
1. Improve cultural practices by ensuring proper fertilization, pruning, and adequate light exposure to enhance plant resistance.
2. Spray a 0.2% to 0.5% copper sulfate solution to control the disease effectively.
Dead Branches (Shoot Blight)
[Causes and Symptoms] This disease is caused by the fungus Glomerella cingulata, which infects shoots and old branches. Affected branches become necrotic, and leaves change from green to yellow, eventually withering and dying from the top down. The shoot tip may die completely.
[Disease Cycle] As temperatures rise in spring, the pathogen in overwintering dead wood becomes active. It spreads through wind, entering through wounds, leaf scars, grafts, or pruning cuts. Once inside, the fungus multiplies and causes damage.
[Control Methods]
1. Cut off and burn dead branches in winter to reduce inoculum.
2. Remove unnecessary shoots and thin weak branches to limit the spread of the pathogen.
3. Isolate infected plants to prevent transmission.
4. Before bud break, apply fungicides like thiophanate-methyl, thiram, or chlorothalonil, especially on wounded areas.
5. Use more phosphorus and potassium fertilizers, and reduce nitrogen application to strengthen the plant's resistance.
Root Rot
[Causes and Symptoms] Root rot is caused by various fungi, including Phytophthora cinnamomi, Cylindrocladium crotalariae, and Armillaria. These pathogens attack the roots, causing them to rot and darken. Infected plants show yellowing leaves, dead leaf tips, dropped flower buds, and eventual wilting and death.
[Disease Cycle] The pathogen overwinters in soil or diseased plants as sclerotia. In spring, under favorable temperature and moisture conditions, the sclerotia germinate and infect the roots. Higher temperatures increase the likelihood of infection.
[Control Methods]
1. Dig out infected plants and surrounding soil, and treat the area with a 1% copper sulfate solution to prevent further spread.
2. Ensure good drainage in the planting area to avoid waterlogging.
3. Choose disease-resistant camellia varieties for planting.
Red Leaf Spot (Camellia Leaf Spot)
[Causes and Symptoms] This disease is caused by fungi of the genus Phyllosticta. It primarily affects young leaves, starting with small, light brown spots that expand into larger tan lesions. In severe cases, the spots merge, causing significant leaf browning and shedding.
[Disease Cycle] The pathogen survives on infected plant debris. Spores are dispersed by wind and rain. The disease typically starts in May and peaks between July and September, leading to extensive leaf loss.
[Control Methods]
1. Spray with 70% thiophanate-methyl diluted at 1,000–1,500 times, or 25% mancozeb at 400 times, for prevention and treatment.
2. Increase watering during dry periods to help suppress disease development.
Flower Rot
[Causes and Symptoms] Flower rot is caused by the fungus Giborinia camelliae, which infects petals. Infected flowers develop small brown spots that gradually enlarge, turning the entire flower brown and causing it to wither prematurely.
[Disease Cycle] The fungus forms sclerotia at the base of the flower and spreads via wind. The disease is more prevalent in autumn but becomes active again in late winter to early spring as temperatures rise.
[Control Methods]
1. Immediately remove and destroy any infected flowers to prevent the spread of spores.
2. Before flowering, spray buds with fungicides like carbendazim, applying 2–3 treatments.
3. Select fall-blooming camellia varieties or use gibberellin to delay blooming, helping the plant avoid peak disease periods.
4. Use biological control methods, such as inoculating the soil with Sporidesmium sclerotivorum and Coniotryum minitans, which can kill the pathogen's spores.
5. Avoid purchasing or cultivating plants that show signs of flower rot.
Bituminous Coal Disease
[Causes and Symptoms] This disease is caused by the presence of aphids and scale insects, which secrete honeydew that allows the growth of sooty mold. A black, tar-like layer forms on the leaves, interfering with photosynthesis and gas exchange. This severely hinders the plant’s growth and development.
[Environmental Factors] The pathogen thrives in cool, humid conditions, with optimal temperatures ranging from 10°C to 20°C. Higher temperatures within this range accelerate the spread of the disease. Long-term weed infestations, high humidity, and poor light conditions in camellia forests also favor the disease’s development.
[Control Methods] Since aphids and scale insects are the primary vectors, controlling these pests is essential for managing bituminous coal disease. For detailed strategies on controlling these pests, refer to the section on "Main Insect Pests and Their Prevention." Eliminating the pests naturally helps eliminate the disease as well.
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