This has been another “interesting” year for corn planting. Wet conditions have delayed planting in many regions of Missouri. Some corn that was planted early is being replanted because of poor stands. Some planted fields have been flooded and some of these may still be flooded. Fields in which young corn plants are subjected to saturated soil conditions for 24-48 hours from planting to about the five-leaf stage of growth are most likely to develop symptoms of crazy top. Fields that are flooded or saturated for longer periods of time may suffer more damage from the effects of flooding and oxygen deprivation than from diseases such as crazy top.
Crazy top of corn is caused by the downy mildew fungus, Sclerophthora macrospora. The causal fungus is a soilborne fungus, which causes infection when young plants are subjected to saturated soil conditions or water accumulating in whorls or leaf sheaths.
In corn, crazy top is likely to occur when young corn plants are subjected to saturated soil conditions for 24-48 hours from planting to about the five-leaf stage of growth. Accumulation of soil and water in the whorl of small plants may also result in infection. The disease causes a deformation of plant tissues including excessive tillering, rolling of leaves, proliferation of the tassel until it resembles a mass of leafy structures and stunting of corn plants. Leaves of infected plants may be narrow and straplike in shape, leathery in texture and yellow or yellow striped in color.
In seasons with wet springs or rains after corn has emerged, young corn plants subjected to saturated soil conditions may show symptoms of crazy top. Occasionally a band of affected plants may encircle a drowned out spot in a field. Some hybrids may be more susceptible to crazy top. This disease is seldom severe enough to cause significant losses.
The downy mildew fungus which causes crazy top of corn (Sclerophthora macrospora) has been reported on more than 140 species of perennial and annual grasses. In addition to corn, downy mildew occurs on wheat, barley, rice, oats, sorghum, crabgrass, green foxtail, barnyard grass and numerous other grasses. In addition to surviving in various grass hosts, the fungus produces survival structures called oospores which can persist for months in infested crop residues and in the soil.
Losses from crazy top are seldom severe enough in corn to warrant control. Furthermore practical management options for crazy top are very limited. Improving soil drainage or water management may be beneficial. Rotation to nongrass crops may help may also be of some benefit.