一、霜霉病1.发病症状此病为常发性病害,春秋两季发生。苗床开始发病时,下部叶片出现褪绿斑,逐渐变为淡黄色,中央略带黄褐色稍凹陷,并受叶脉限制呈多角形病斑,湿度大时叶背或叶面生稀疏的白色霉状物,病斑连片后呈黄褐色,病重时病斑连片,叶片枯萎。气温8～16℃、空气相对湿度98%以上、过度密植、偏施 First, downy mildew 1. Symptoms The disease is a common disease, occurs in spring and autumn. When the seedbed began to develop, the lower leaves appeared chlorotic spots, becoming gradually yellowish, with a slightly brownish central slightly depressed, and restricted by the veins polygonal lesions, when humidity is high, leaves sparse or foliar sparse white mildew Objects, lesions contiguous was brown, diseased lesions contiguous, withered leaves. Temperature 8 ~ 16 ℃, air relative humidity above 98%, over close planting, partial Shi