Detailed Explanation of the Occurrence Characteristics and Control Methods of Leaf Blight

During the plum rain season in Jiangnan, when new and veteran orchid growers alike turn pale at the mere mention of "rot" (soft rotdisease), they often overlook a chronic killer - leaf blight. Generally, small reddish-brown spots first appear on orchid leaves, then rapidly expand into semicircular, circular or irregular water-soaked brown patches, which are often mistaken for the more common anthracnose and dismissed. In fact, these are the early symptoms oforchidleaf blight. Of course, sometimes no lesions form, and instead entire sections discolor and gradually die back downward (as shown in Figures 1 and 2). The lesions are relatively large in diameter, and in later stages there may be a dark brown band at the edges, though sometimes the boundary is unclear. The affected areas gradually turn grayish-white or grayish-brown. When lesions are numerous or persist for a long time, they may also merge with one another, and the affected areas gradually lose chlorophyll or turn yellow, causing large areas of orchid leaves to die (as shown in Figures 3 and 4). Dark brown dots appear on the undersides of dead leaves. The main difference between this disease andorchidanthracnose lies in the color and arrangement of the dot openings: the dot openings of leaf blight are white and arranged in a sparse wheel-like pattern.Orchidleaf blight mainly damages theCymbidium goeringii、Cymbidium ensifolium、Cymbidium sinense、Cymbidium kanranand other orchidplants'leaves, with the middle and lower leaves of the plant being particularly severely affected. Symptoms vary with air humidity: when humidity is especially high, lesions rapidly expand and bulge in a yellowish-brown color, with the epidermis lifting to form blisters. At moderate humidity, lesions secrete a yellowish-white gelatinous substance. The pathogens of leaf blight are: 1. Cylindrosporium phalaenopsidis Saw., called Phalaenopsis cylindrosporium; 2. C. dioscoreae, called yam cylindrosporium; 3. C. padi, called Prunus cylindrosporium. They overwinter or oversummer on diseased leaves as acervuli, conidia or mycelium. When temperature and humidity are suitable, they produce conidia, which spread and propagate through wind and rain or water droplets splashing during watering. They mainly invade through wounds, but can also directly infect young leaves. The incubation period is generally 7 to 30 days. In the Jiangsu-Zhejiang area, the disease period for old leaves is from April to July, with May to June being the peak incidence period. The disease period for new leaves is from July to November, and new leaves suffer more severe damage than old leaves. In the Guangdong area, the disease can begin in December, peak in March, and gradually decline to stability after May. Leaf blight is prone to occur when relative humidity is high, and is more severe when flower pots are placed too densely, light and ventilation are poor, or the potting soil is compacted. Therefore, the focus of leaf blight control should be on prevention first. The first priority is ventilation and light penetration, because sunlight is the best and most natural physical sterilization method. Second, diseased and dead withered leaves should be promptly pruned and burned to reduce sources of infection and prevent the disease from spreading further. Finally, during the disease period, effective chemicals should be selected for chemical control. The following can be used: Longkejun 500x solution, Aimiao 3000x solution, Cuibei 2000x solution, sprayed regularly for prevention and control. Excerpted from "Orchid Treasure Classic" Author: Yu Junjie, Shanghai