How to prevent and control bacterial leaf spot in geraniums?

Geranium, also known as zonal geranium, fish geranium, and horseshoe geranium. It is native to South Africa. It prefers cool conditions, fears high temperatures, and is not cold-tolerant. It requires ample sunlight. It is not tolerant of waterlogging but is slightly tolerant of drought, and prefers fertile loam with good drainage. In spring and autumn, when the weather is cool, it is most suitable for the growth of geraniums. In winter, indoors at around 15°C during the day and no lower than 5°C at night, with sufficient light, they can bloom continuously. In hot summers, the plants enter a dormant or semi-dormant state, and should be placed in partial shade with attention to controlling watering. It is resistant to chlorine. Pelargonium plants are important potted flowers. They are extremely commonly cultivated. There are 2 types of varieties: flowering and foliage. They are commonly used as spring and summer flowerbed material in Beijing, Shanghai, Northeast China, and other places. They are commonly used flowers for flowerbed arrangements during the "May Day" holiday. In regions with warm winters and cool summers, they can be cultivated outdoors year-round. Scented geranium, rose-scented geranium, and fragrant geranium, among others, can be used to extract essential oils for the cosmetics and soap industries, and are often cultivated in large fields as cash crops.

Bacterial leaf spot of geranium is one of the important diseases of geranium. Most of the leaves of infected plants die, causing defoliation, which affects the growth and flowering of geraniums and reduces the ornamental value of the plants.

Symptoms: This disease mainly affects the leaves, and sometimes also infects the stems of the plants. In the early stage of the disease, the affected leaves produce spots the size of a pinhead. The lesions are round or irregular in shape, brown and sunken, with faint concentric rings. When the disease is severe, the diseased leaves are covered with spots. There are more spots on the older leaves at the lower part. Later, the lesions expand and gradually merge together, causing most of the leaf to die. Finally, the diseased leaves fall off, the stems develop black rot, and the stems of infected plants turn entirely black, wilt, and rot. When the stem is cut open, the vascular bundles can be seen turning brown and black. The disease is very severe on cutting seedlings; the cuttings begin to rot from the base and the rot develops upward until the leaves wilt and die.

Pathogen: The pathogen is the bacterium Xanthomonas pelargonii (Brown) Starr et Burkholder.

Disease pattern: The pathogen survives in the diseased parts of the plant and spreads through contact, splashing water droplets, and cuttings. It can survive in moist soil for 3 months. The disease is severe in greenhouses with high densities of young plants and excessively high temperatures, and is especially severe when the weather is warm and humid. In such weather, even geraniums in gardens have many leaf spots.

Control: ① Reduce sources of infection: Remove all infected leaves and destroy them in a centralized manner. ② Strengthen cultivation management: Avoid overhead irrigation to prevent spreading bacteria from one plant to another. Adjust ventilation conditions and reduce humidity. Plants should not be planted too densely. Avoid splashing diseased soil onto healthy plants. ③ Chemical control: In the early stage of the disease, spray agricultural streptomycin for prevention, once every 10-15 days, spraying 2-3 times.