Characteristics and control of leaf blight

In the mildew and rain period in the south of the Yangtze River, when the vast number of new and old orchid friends talk about "rot" (soft rot), a chronic killer-leaf blight is often omitted.

In general, small reddish-brown spots are born in orchid leaves, followed by rapid expansion of semicircular, round or irregular waterlogged brown spots, which are often mistaken for anthrax, which is actually the early symptom of orchid leaf blight. Of course, sometimes the disease spot is not formed, but the whole discoloration gradually dies down (figure 1, 2). The diameter of the disease spot is larger, and there will be a dark brown color band on the edge of the later stage, but sometimes the boundary is not clear. The affected area gradually became grayish white or grayish brown. When the number or duration of disease spots is long, the disease spots will merge with each other, and the damaged parts will gradually lose green or yellowing, resulting in large area withering of orchid leaves (Fig. 3, 4). Dark brown dots appear on the back of dead leaves. The main difference between this disease and orchid anthracnose lies in the color and arrangement of the opening. The opening of leaf blight is white and arranged in a sparse wheel. Orchid leaf blight mainly harms the leaves of Cymbidium, Jian Orchid, Mulan, Cold Orchid and other orchids, especially the middle and lower leaves of the plant. Symptoms vary with air humidity. When the humidity is particularly high, the plaque expands rapidly and bulges in yellowish brown, and its epidermis warps up to form blisters. When the humidity is moderate, the plaque will secrete yellow-white gelatinous material.

The pathogen of leaf blight is 1 Cylindrosporium phalaenopsidis Saw. Phalaenopsis is called Phalaenopsis 2, C.dioscoreae is called Dioscorea 3, C. padi becomes Prunus. The conidia will survive the winter or summer on the diseased leaves as conidia, conidia or mycelium. When the temperature and humidity are suitable, the conidia will spread and spread with the splash of water droplets during wind, rain or watering. It is mainly invaded by the wound and can also be directly infected to the young leaves. The incubation period is generally 7-30 days. In Jiangsu and Zhejiang, the disease onset period of old leaves is from April to July, and from May to June, it can enter the high incidence period. The disease period of new leaves is from July to November, and the damage of new leaves is more serious than that of old ones. In Guangdong, the disease began in December, peaked in March, and slowed down to stable after May. Leaf blight is easy to occur when the relative humidity is high, and the disease is more serious when flowerpots are placed too densely, poor light transmission and ventilation or basin soil consolidation.

Therefore, the prevention and control of leaf blight should focus on the first prevention. First of all, it should be ventilated and transparent, because sunlight is the best and most natural physical sterilization method; again, it is found that dead leaves should be cut off and burned in time to reduce the source of infection so that the disease will no longer spread; finally, effective drugs are selected for drug control during the onset of the disease, and can be used: Longke bacteria 500x liquid, love seedling 3000 times liquid, Cuibei 2000 times liquid regular spraying for prevention and control. Excerpt from the Treasure Book of Orchids by Yu Junjie of Shanghai