By Alison Robertson, Department of Plant Pathology
Corn across the state is beginning to tassel and many of you, like me, have been out in fields scouting for foliar diseases of corn.
The disease of this year, thus far, seems to be eyespot, particularly in central and northern Iowa. Eyespot loves wet and cool weather so it is probably not surprising that the disease is especially prevalent this year. If such weather continues, we can expect the disease to spread.
Eyespot tends to be more severe in corn following corn fields, especially if residue is present on the surface. Older corn leaves appear to be more susceptible to infection. The disease usually starts on the lower leaves of the plant and moves up the plant; however, random lesions may also occur in the upper leaves. Hybrids do vary in their susceptibility to the disease.
Initial symptoms are small (one-sixteenth inch in diameter), water-soaked, circular lesions. The lesions enlarge to about one-eighth inch in diameter, and develop a light tan center, dark brown margin and yellow "halo" (photo below). If you hold the leaf up to the light, the halo becomes more apparent, and the center of the spot looks translucent. Time from infection to symptom development is about 9-10 days.
Early and severe eyespot can result in yield loss on susceptible hybrids. Furthermore, increased stalk rot severity is associated with severe eyespot disease.
Gray leaf spot
Gray leaf spot (GLS) is starting to show up on the lower leaves of susceptible hybrids across the state. Tamra Jackson, Extension Plant Pathologist at University of Nebraska — Lincoln, reported GLS is highly prevalent at this time in Nebraska, occurring slightly earlier than normal in that state, and the severity is increasing. Here in Iowa, the disease is perhaps a little ahead of schedule. We usually see a few lesions of the GLS on the lower leaves of the plant around mid to late July. GLS infections in June and early July were associated with the epidemics of GLS in Iowa in the mid 1990s.
GLS disease development is favored by is very warm, humid weather ("Iowa State Fair" weather), and the disease is usually more severe when the previous crop was corn and residue is present on the surface. Like eyespot, GLS usually starts on the lower leaves of the plants and progresses up the plant.
Lesions of GLS are rectangular in shape (photo below) since the veins of the leaf stop the lesions expanding sideways. The lesions may develop up to 4 inches in length. Lesions are initially tan to brown but become silvery-gray when they produce spores. Time from infection to symptom development is about 14-21 days, depending on weather conditions and hybrid susceptibility.
GLS can result in considerable yield loss particularly when the disease spreads to leaves above the ear soon after tasseling. High GLS severity may also increase the risk of stalk rot.
Common rust can also be found in Iowa corn fields this year but at extremely low incidence. Common rust does not survive the winter in Iowa and spores blow into the state from the south each year. This time last year, common rust could be easily found in every corn field.
The cornerstone of eyespot and GLS management is resistance. Rotation can also reduce risk of disease. Fungicides are also effective at reducing disease and protecting yield.
To determine if a fungicide application is necessary, the following factors should be considered:
• disease pressure in the field
• hybrid susceptibility
• predicted weather conditions during grain fill
• price of corn and cost of fungicide plus application
• previous crop
• field history of disease
Small, circular lesions of eyespot. Note the yellow halo, dark brown ring and light tan center.
Characteristic rectangular gray leaf spot lesions.
Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases. Robertson may be reached at (515) 294-6708 or by email at email@example.com.