How to prevent and control bacterial leaf spot in geraniums?

Geranium, also known as zonal geranium, fish geranium and horseshoe geranium. Native to South Africa. It prefers cool conditions, fears high temperatures, and is not cold-tolerant either. It requires ample sunlight. It is not tolerant of waterlogging but is slightly tolerant of drought, and prefers well-drained, fertile loam. In spring and autumn, when the weather is cool, conditions are 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 maintained, they can bloom continuously. In hot summers, the plants enter a dormant or semi-dormant state, so they should be placed in partial shade and watering should be carefully controlled. It is resistant to chlorine. Plants of the genus Pelargonium are important potted flowers. Cultivation is extremely common. There are 2 types of varieties: those grown for flowers and those grown for foliage. In Beijing, Shanghai, Northeast China and other places, they are commonly used as spring and summer flowerbed material. They are flowers commonly used in flowerbed displays for the "May 1st" holiday. In regions with warm winters and cool summers, they can be grown 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 grown on a large scale as an economic crop.

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 geranium and reduces the ornamental value of the plant.

Symptoms: This disease mainly damages leaves, and sometimes also infects the stems of the plant. In the early stage of the disease, the affected leaves develop spots the size of a needle head. The lesions are circular or irregular, brown and sunken, with slightly visible concentric rings. When the disease is severe, the diseased leaves are covered with lesions. 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. This disease is very severe on cuttings; 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 development: The pathogen survives in the diseased parts of the plant and is spread by contact, splashing water droplets and cuttings. It can survive in moist soil for 3 months. The disease is severe in greenhouses with high seedling density 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.

Prevention and control: ① Reduce sources of infection: remove all infected leaves and destroy them centrally. ② Strengthen cultivation management: avoid sprinkler irrigation to prevent the bacteria from spreading 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, for 2-3 sprays.