Minecraft wiki entry for Mushroom_farming, associated text: Since the Java Edition Beta 1.6 update, mushrooms will slowly spread to nearby opaque blocks, allowing for the creation of farms. However, the rate at which mushrooms spread is considerably slow; therefore, starting a farm is an ordeal without multiple mushrooms.
As of Java Edition Beta 1.8, mushroom spreading is nerfed, so that a mushroom will only spread if there are fewer than 5 mushrooms of the same type in a 9x9x3 area around the original mushroom. This change made Mushroom farming less efficient and many of the former methods (most notably the "water flushable pentapipe") will not work in 1.8+. However, the addition of huge mushrooms provides a faster alternative.