By Fanny Iriarte and Alison Robertson, Department of Plant Pathology
The Plant Disease and Insect Diagnostic Clinic recently received some corn samples with symptoms and signs of crazy top, a disease caused by Sclerophthora macrospora.
Symptoms of this disease vary greatly with time of infection and degree of host colonization by the pathogen. Generally, excessive tillering, rolling, and twisting of the upper leaves appear first. Later, infected plants produce a "leafy" tassel.
Crazy top develops when soils have been flooded shortly after planting or before plants are in the four to five leaf stage. Pooling of soil and water in the whorl of small plants can also lead to infection. Saturation for 24-48 hours is sufficient for infection to occur, since it allows the overwintering soilborne oospores to germinate and produce zoospores that swim in the water to infection sites on corn.
Diseased grasses may also be a source of inoculum. According the the Compendium of Corn Diseases infection occurs at a wide range of soil temperatures. Although seed transmission of the pathogen has been demonstrated from freshly harvested seeds, it is generally considered unimportant in the spread of the pathogen.
Crazy top is not a severe disease but it can cause significant losses in low-lying areas of fields that are prone to flooding, since infected plants are often barren. This disease is best managed by improving field drainage or by avoiding low, wet areas in fields. Rotation may be helpful if care is taken to keep rotational fields free of grassy weeds.
Therefore, there's not much that can be done to manage this disease in season. Pulling up and discarding infected plants can reduce buildup of inoculum in the field, and thus reduce the risk of disease in subsequent years.
Fanny Iriarte is a plant pathologist at the Plant Disease and Insect Diagonostic Clinic. Alison Robertson is an assistant professors of plant pathology with research and extension responsibilities in field crop diseases. Iriarte may be contacted at (515)294-5374 or by emailing firstname.lastname@example.org. Robertson may be contacted at (515)294-6708 or by emailing email@example.com.