发病原因大白菜霜霉病属真菌性病害。以叶片发病为主。茎、花及种荚地也能受害。感病以后叶正面产生水渍状褪绿斑,后发展为黄褐色,由于病斑扩大常受叶脉限制而呈多角形斑。天气潮湿时,叶背面产生白色稀疏的霉层,病斑连片时,常造成大白菜叶片变黄枯死,严重影响大白菜包心。低温多雨、温度忽高忽低、昼夜温差大、露水大易发生霜霉病。 The cause of cabbage downy mildew is a fungal disease. The main blade disease. Stems, flowers and seed pods can also suffer. After the onset of the leaves positive water stains like chlorotic spot, after the development of brown, due to enlargement of the lesion is often limited by the veins and was polygonal plaque. When the weather is wet, white sparse moldy layer is formed on the back of the leaves. When the lesion is contiguous, the cabbage leaves often become yellow and dead, which seriously affects the cabbage heart. Low temperature and rainy, the temperature fluctuated, the temperature difference between day and night, dew big prone to downy mildew.