１症状发病初期叶片正面产生淡绿色病斑，后逐渐扩大成多角形或不规则形，颜色由淡绿色转为黄至黄褐色，病斑背面长出明显的白色霜霉。包心期若环境适宜，病斑可迅速扩大，使叶片连片枯死，从植株外叶向内叶层层发展、干枯，最后剩下一个叶球，造成大白菜严重减产。２病原及传 1 Symptoms Early onset of leaves produce light green lesions, and gradually expanded into polygons or irregular shape, the color from light green to yellow to brown, the back of the lesion grow obvious white downy mildew. If the environment at the end of the pericardium is suitable for the environment, the lesion can be rapidly expanded, making the leaves withered, from the outer leaves of the plant to the development of the inner leaf layer, dry, and finally left a leaf ball, causing serious cabbage production. 2 pathogen and pass