うどんこ病
Powdery mildew was first reported in European grape vineyards in the mid-1850s. It could reduce yields by up to 80%. In France alone it reduced wine production from a billion gallons to only two hundred million gallons. It was as a result of this disease's rapid spread throughout the continent that the first fungicide, sulfur, was discovered. The success of sulfur was so great that the grape industry was sparked in California when it was seen that disease could now be controlled and grapes were easy to grow. Uncontrolled powdery mildew rots the grapes from the inside out. Powdery mildew slows the growth of the grape skin so that the inside pulp grows at a faster rate and bursts through. These split grapes dry up and rot. The grapes that do not split open are of much lower quality than normal grapes. Their acidity is much higher and as a result the wines taste bitter and off to most wine drinkers. In order to control powdery mildew the same treatment that was recommended in the late 19th century is still used today: regular applications of sulfur every 1–2 weeks.