What are the main diseases affecting camellia plants and how can they be prevented?
Anthrax
[Causes and Symptoms] Anthrax is a common disease in camellia plants caused by the fungus Colletotrichum gloeosporoides. It affects approximately 33% of camellia crops. The disease typically starts on the leaf margins, tips, and veins, with dark green spots that gradually expand into irregular, larger lesions. Over time, the color changes from brown to black, and in severe cases, the entire leaf may fall off.
[Occurrence] The disease thrives in temperatures between 25°C and 28°C, especially during periods of high humidity and continuous rainfall. It usually begins in April, peaks between June and July, and declines after September.
[Control Methods]
1. Remove infected leaves promptly.
2. Prune dead branches during winter.
3. Apply a 1% Bordeaux mixture every two weeks before the start of each season.
4. Spray with a 600-fold solution of chlorothalonil once a week for 3–4 consecutive times.
5. Improve cultivation practices, including 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] This condition is caused by 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 develop a fine, felt-like texture. As the disease progresses, the color changes to dark brown, reducing the plant's ability to photosynthesize and weakening its growth.
[Pathogenesis] The algae overwinter within infected tissues and produce zoospores in wet conditions. These spores infect new plants, especially in poorly ventilated and shaded areas. High temperature and humidity favor the spread of the disease.
[Control Methods]
1. Improve cultural practices, such as proper pruning, fertilization, and ensuring good air circulation.
2. Spray with a 0.2% to 0.5% copper sulfate solution to control the infection.
Dead Branches
[Causes and Symptoms] This disease is caused by the fungus Glomerella cingulata, which infects shoots and old branches. Infected branches turn necrotic, and the leaves change from green to yellow, eventually withering and dying from the top down.
[Occurrence] As temperatures rise in spring, the pathogen becomes active, spreading through wounds on shoots, leaves, grafts, or pruning sites. It invades and multiplies, causing damage.
[Control Methods]
1. Cut and burn dead branches in winter.
2. Remove weak and unnecessary branches to reduce disease sources.
3. Isolate diseased plants.
4. Before budding, apply fungicides like thiophanate-methyl, thiram, or chlorothalonil, especially on wounds.
5. Use more phosphorus and potassium fertilizers, and limit nitrogen application.
Root Rot
[Causes and Symptoms] Root rot is caused by various fungi, including Phytophthora cinnamoni, Cylindrocladium crotalariae, and Armillaria. These pathogens attack the roots, causing them to rot and darken. Affected plants show yellowing leaves, dying tips, dropped flower buds, and eventual death.
[Pathogenesis] The pathogens survive in the soil or on infected plants during winter. In spring, under favorable conditions, the spores germinate and infect the roots. Higher temperatures increase the risk of infection.
[Control Methods]
1. Remove affected plants and sterilize the surrounding soil with a 1% copper sulfate solution to prevent further spread.
2. Ensure good drainage to avoid waterlogging in the planting area.
3. Choose resistant camellia varieties for cultivation.
Red Leaf Spot (Rice Blight)
[Causes and Symptoms] This disease is caused by fungi of the genus Phyllosticta. It primarily affects tender leaves, starting with small, light brown spots that grow into tan or larger patches. Severe cases lead to extensive leaf spotting and shedding.
[Pathogenesis] The pathogen lives on infected plants, and spores are spread by wind and rain. The disease usually starts in May, peaks between July and September, and causes significant leaf loss.
[Control Methods]
1. Apply a 70% thiophanate-methyl solution at 1,000–1,500 times dilution, or 25% mancozeb at 400 times dilution, for prevention and treatment.
2. Increase watering during dry periods to suppress disease development.
Flower Rot
[Causes and Symptoms] Flower rot is caused by the fungus Giborinia camelliae, which infects petals. Initially, small brown spots appear, which then expand until the entire flower turns brown and withers.
[Pathogenesis] The fungus forms sclerotia at the base of the flower and spreads via wind. The disease is most active from December to March when temperatures rise.
[Control Methods]
1. Immediately remove and burn infected flowers, especially during the flowering period.
2. Apply fungicides like carbendazim to buds 2–3 times before flowering.
3. Plant late-flowering varieties or treat them with gibberellin to shift blooming to autumn, avoiding peak fungal activity.
4. Use biological control agents such as Sporidesmium sclerotivorum and Coniotryrium minitans to kill spores in the soil.
5. Avoid purchasing or cultivating plants showing signs of flower rot.
Bituminous Coal Disease
[Causes and Symptoms] This disease is triggered by aphids and scale insects, which excrete honeydew that supports the growth of black fungal mycelium on the leaf surface. This "bituminous coal" layer interferes with photosynthesis and gas exchange, stunting plant growth.
[Influence] The pathogen thrives in cool, humid conditions (10–20°C), with higher temperatures accelerating its spread. Dense weeds, high humidity, and poor light conditions promote the disease.
[Control Methods] Since aphids and scale insects are the primary vectors, controlling these pests is essential. Effective methods for managing these pests can be found in the section on "Main Insect Pests and Their Prevention." By eliminating the pests, the disease will naturally subside.
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