一、大葱霜霉病 1.症状识别及传播规律 大葱植株生长期被害叶片上产生黄白色卵圆形或圆形病斑,边缘不明显。潮湿时,病斑表面产生一层疏松的白色霉层。后期病斑变成暗紫色,或淡灰黄色。叶片中下部染病时,病部以上叶片下垂而干枯或叶片折倒。病菌主要以卵孢子在土壤 First, the green onions downy mildew 1. Symptoms identification and transmission of the lawn plant growth stage damage caused by the leaves of yellow-white oval or round lesions, the edge is not obvious. Wet, the lesion surface layer of white moldy layer of loose. Later lesions become dark purple, or light gray yellow. Lower leaves in the disease, the disease above the leaves sagging or leaf folds. Pathogenic bacteria mainly oospores in the soil