Chrysanthemums are commonly affected by three major diseases: rust, gray mold, and black spot. These diseases often occur together, which can significantly impact the plant’s flowering and reduce its ornamental value. Over the past few years, we have conducted extensive research on these diseases and now share our findings on their causes, symptoms, and effective prevention and treatment methods. **1. Rust** Rust primarily affects the leaves and stems of chrysanthemums, with the leaves being the most vulnerable part. In the early stages, small yellow spots appear on the leaves, which later turn brown and slightly raised. These are uredia spores of the fungus. As the disease progresses, they rupture and release yellow-brown powder—summer spores. In the later stages, dark brown or black oval-shaped swellings develop on the leaves, petioles, and leaf stems, known as teliospores. Severe infestations can lead to complete leaf death. Factors that contribute to the development of rust include poor cultivation practices, insufficient ventilation, poor drainage in low-lying areas, nutrient-deficient soil (especially low nitrogen), and high humidity levels. **Control Methods for Rust:** 1. **Improve Cultivation Practices:** Ensure proper drainage, especially in areas with high water tables. For potted plants, make sure drainage holes are clear to prevent waterlogging. Apply balanced fertilizers, focusing on phosphorus and potassium to enhance the plant’s resistance to disease. 2. **Remove Infected Parts:** As soon as infected leaves or branches are detected, remove them immediately. Dispose of them by burning or burying to eliminate the pathogen. 3. **Chemical Control:** Once the disease is present, apply a 65% zinc oxide solution diluted at 500 times, or a 0.3–0.5° Baumé sulfur solution. In early spring, before the growing season begins, spray with 4–5° lime sulfur to help control future outbreaks. **2. Gray Mold** Gray mold mainly affects the leaves, stems, and flowers of chrysanthemums. Infected leaves show brown, rim-like wrinkles along the edges, while the petioles and pedicels become soft and eventually rot. When flowers are affected, seed development is hindered. This disease thrives in warm, wet conditions, excessive nitrogen fertilizer use, overcrowded planting, and heavy soil textures. **Control Methods for Gray Mold:** 1. **Use Disease-Free Soil:** The pathogen overwinters in the soil, so it's essential to use clean, new soil for both garden and potted plants. 2. **Remove Diseased Material:** Immediately remove and destroy infected leaves and plants through burning or deep burial to prevent the spread of the disease. 3. **Preventive Treatment:** Before planting, dip new chrysanthemum cuttings in a 65% zinc oxide solution diluted at 300 times for 10–15 minutes. 4. **Improve Air Circulation:** Ensure good ventilation and avoid overwatering. During rainy seasons, keep drainage ditches open. In the early stages of infection, apply a 0.3–0.5° Baumé sulfur solution or fungicides such as zineb or carbendazim. **3. Black Spot** Black spot disease mainly attacks the leaves of chrysanthemums. Initially, small black spots appear on the leaves, which gradually grow larger. The edges turn dark brown, while the center becomes grayish-black. In severe cases, multiple lesions merge, causing the entire leaf to turn black and fall off. This disease is particularly common during rainy periods. **Control Methods for Black Spot:** 1. **Drain Excess Water:** During the rainy season, ensure proper drainage to reduce soil moisture and prevent the spread of the disease. 2. **Prune Regularly:** Remove old, lower parts of the plant to improve air circulation and light exposure, which helps reduce humidity between the foliage. 3. **Remove Affected Leaves:** As soon as black spots are noticed, remove the infected leaves and spray the plant thoroughly with a 0.3–0.5° lime sulfur solution to prevent further spread. By implementing these strategies, chrysanthemum growers can effectively manage these common diseases and maintain healthy, vibrant plants throughout the growing season.

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