MDM is a widespread disease of corn and sorghum in the United States. The causal virus readily infects johnsongrass and numerous other grasses. Perennial weeds also serve as overwintering hosts for the virus. In nature, the causal virus is transmitted from the weed hosts to grain sorghum through the feeding activities of several species of aphids. These vectors feed on infected weed hosts in the spring and acquire the virus.
Plants produced by johnsongrass rhizomes are a common source of virus.
The aphids then move to nearby crop plants and transmit the virus to them. If weed hosts are controlled in fields, but are populous along field perimeters, then border plants will often be the first infected. Later, buildup and dispersal of aphids both within and between fields results in extensive spread of the disease. At this time, a distinct yellowing and mottling of leaves, especially young leaves, will be noticeable. In late July to early August, the leaves will begin to develop a reddish coloration and elongated stripes with necrotic centers and reddish margins.