Diseases are already showing up in Iowa corn

Diseases are already showing up in Iowa corn

Within the past week several foliar diseases have been reported in Iowa cornfields.

Northern corn leaf blight (NCLB) has been reported already this year in several fields in southern Iowa. This disease (see Photo 1) was widespread in Iowa in 2014, and was severe on susceptible corn hybrids. "Since the fungus survives the winter in corn residue, we likely have above normal inoculum present," says Alison Robertson, Iowa State University Extension plant pathologist.

LEAF DISEASES: Southern and central Iowa continue to see an earlier than typical occurrence of a leaf disease, northern corn leaf blight, in some cornfields. "I haven't seen this in northeast Iowa yet this summer, but I should step up my scouting," says ISU agronomist Brian Lang.

Cool weather with frequent precipitation favors infection of corn by the fungus and disease development, says Robertson. New lesions may develop every four days when conditions are favorable. That is, a susceptible hybrid, and cool and wet weather. Warm dry conditions will slow or halt disease development until favorable conditions return.

Northern corn leaf blight: It will be very important this growing season to scout fields that are planted to NCLB-susceptible hybrids, says Robertson. If the disease is present on 50% or more of the plants in the field, the hybrid is scored susceptible and cool, wet weather is forecast, a foliar fungicide application may be required.

In 2014, applications at V5-V6 reduced NCLB, but applications made at R1 growth stage of corn were more effective at protecting the canopy through dent. In 2014 however, July was dry and NCLB development slowed or stopped before starting up again in August through September. If 2015 remains cool and wet, NCLB will win the "Disease of the Year" award for a second consecutive year.

Anthracnose leaf blight: On corn-following-corn fields, many farmers and crop advisers have likely noticed anthracnose leaf blight (see Photo 2) on the lowest leaves of the corn plant. This is not unusual, says Robertson.  Anthracnose leaf blight is usually present in wet springs, but does not need to be managed.

Research at Iowa State University and University of Wisconsin has shown that there is no relationship between anthracnose leaf blight and anthracnose stalk rot, although both are caused by the same pathogen. "Corn will rapidly grow out of the disease, and the affected lower leaves, which do not contribute to yield, will die and fall off the plant within a couple of weeks," she notes.

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Common rust: This disease (see Photo 3) has also been observed this growing season but at very low incidence (few plants in a field). Most corn hybrids have good resistance to common rust. However, corn inbreds do not. Thus seed production fields should be scouted and a fungicide applied if this disease is present.

Southern and central Iowa continue to see an earlier than typical occurrence of the foliar disease, northern corn leaf blight, in some cornfields. "I haven't seen this in northeast Iowa yet this summer, but I should step up my scouting," says ISU Extension field agronomist Brian Lang, based at Decorah. "The disease overwinters on corn residue. Continuous no-till corn would be at greater risk for a reoccurrence of the problem this season assuming the weather is favorable for disease development." 

Favorable weather includes above normal rainfall and below normal temperatures and long periods of dew, says Lang. The ideal temperature for this disease is around 68 degrees F. The presence of this disease is usually not noticed until after tassel stage (the VT stage of corn growth). He adds, "If you are scouting at this time and you find 50% or more of plants with a NCLB lesion in a field, and the corn hybrid has a known susceptibility to NCLB, then a foliar fungicide application may be warranted."

Alison Robertson is an associate professor of plant pathology with research and Extension responsibilities in field crop diseases at Iowa State University. She may be reached at [email protected] or 515-294-6708.

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