一、危害性苜蓿霜霉病是遍布全球苜蓿种植地区的一种病害。据文献报道,多发生于温和、湿润的气候条件下。但据我们观察,在西北高原的干旱和半干旱地区,此病也经常爆发成灾。例如,1984年9—10月,笔者在甘肃省榆中县北山地区兰州牧研所试验基点的试验地(小区和大田),见此病严重流行。西北常用的一些品种(或地方品系),如和田苜蓿、沙湾苜蓿、天水苜蓿、定西苜蓿等,株发病率几近100%,枝发病率也高达44—80%, First, the harmful alfalfa Downy mildew is a disease all over the world alfalfa planting areas. According to the literature, it occurs mostly in mild and humid climates. However, according to our observations, the disease has also often erupted in arid and semi-arid areas of the Northwest Plateau. For example, from September to October 1984, I tested the disease at a pilot base (plot and field) of the Lanzhou Animal Husbandry Research Institute in Beishan, Gansu Province. Some common varieties of Northwest China (or local strains), such as Wada alfalfa, Shawan alfalfa, Tianshui alfalfa, Dingxi alfalfa, the incidence of nearly 100% strains, the incidence of branches as high as 44-80%