大白菜三大病害是指白菜病毒病、霜霉病和软腐病。这些病害的发病时间长、发病率高、抗药性强,对大白菜安全生产危害极大。识别其发病症状,并做到及时防治,是夺取大白菜优质高产的关键。一、三大病害的发病症状和发生特点1、病毒病多发生在幼苗期。苗期发病心叶呈明脉或叶脉失绿,后产生浓淡不均的绿色斑驳或花叶,叶片皱缩不平,重病苗矮化以致死亡。成株期发病,感病早的植株发病重,叶片严重皱缩,硬而脆,常生许多褐色小斑点,叶背主、侧脉上生褐 The three diseases of Chinese cabbage are cabbage virus, downy mildew and soft rot. The incidence of these diseases a long time, high incidence, strong resistance, safety of cabbage great harm. Identify the symptoms and timely prevention and control is the key to high quality and high yield of Chinese cabbage. First, the symptoms of the three major diseases and the occurrence of symptoms 1, virus disease occurred in the seedling. Emergence of seedling heart leaf or vein veins chlorosis, uneven after the emergence of uneven green mottled or mosaic, leaf folds uneven, sick dwarf disease and death. The incidence of adult plants, early onset of disease incidence of serious leaves serious shrinkage, hard and brittle, often many small brown spots, leaf dorsal, lateral veins brown