茼蒿霜霉病主要为害叶片。发病初期在植株下部老叶上产生淡黄色近圆形或多角形病班,逐渐向植株中上部蔓延,后期病班为黄褐色,病重时病班连成一片,叶片发黄枯死。湿度大时,病班背面产生白色霉层。病菌侵染适宜温度15～17℃,需要85%以上的相对温度。昼夜温差大、结霜时间长或雾多、阴雨等气候条件,是病害易发生时期。另外, Chrysanthemum downy mildew mainly damage leaves. Early onset in the lower leaves of plants produce pale yellow near-circular or polygonal disease classes, and gradually spread to the upper part of the plant, the late disease class was brown, sick disease even into a piece of disease, the leaves yellow dead. Humidity, white mold layer on the back of the sick class. Pathogen infection appropriate temperature 15 ~ 17 ℃, the need for more than 85% of the relative temperature. Large temperature difference between day and night, frost a long time or more fog, rain and other climatic conditions, is prone to disease. In addition,