莴苣是早春上市较早的蔬菜,近年来种植面积不断扩大。但随着种植年限增加,霜霉病等病害发生也逐年加重。莴苣霜霉病主要危害叶片。发病初期植株下部叶片产生淡黄色近圆形或多角形病斑,叶 Lettuce is an early spring vegetables listed in recent years, growing area planted. However, with the increase of planting years, the incidence of downy mildew and other diseases also increased year by year. Lettuce downy mildew is the main harming leaf. In the early stages of the disease, the lower leaves of the plants produce pale yellow, nearly circular or polygonal lesions and leaves