By Alison Robertson, Department of Plant Pathology and Microbiology
Northern corn leaf blight
Northern corn leaf blight (NCLB) (Figure 1) has been reported from several fields in southern Iowa. You will remember that this disease was widespread in Iowa in 2014, and severe on susceptible hybrids. Since the fungus survives the winter in corn residue, we likely have above normal inoculum present. Cool weather with frequent precipitation favors infection of corn by the fungus and disease development. New lesions may develop every 4 days (Muiro et al., 2010) when conditions are favorable (susceptible hybrid, cool and wet weather). Warm dry conditions will slow or halt disease development until favorable conditions return.
Photo 1. Northern leaf blight lesions are large, cigar-shaped, gray becoming tan.
It will be very important this growing season to scout fields that are planted to NCLB-susceptible hybrids. 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 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 of you have likely noticed anthracnose leaf blight (Figure 2) on the lowest leaves of the corn plant. This is not unusual. 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.
Photo 2. Anthracnose leaf blight is common on the lowest leaves of corn plants in corn following corn fields.
Common rust (Figure 3) has also been observed but at very low incidence (few plants in a field). Most hybrids have good resistance to common rust; inbreds do not, thus seed production fields should be scouted and a fungicide applied if disease is present.
Figure 3. Pustules of common rust.
Alison Robertson is an associate professor of plant pathology with research and extension responsibilities in field crop diseases at Iowa State University. Alison Robertson may be reached at firstname.lastname@example.org or 515-294-6708.