If you grow corn, keep an eye out for northern corn leaf blight. It's already showing up in a number of Iowa cornfields this summer, says Alison Robertson, Iowa State University Extension plant pathologist. She wrote an article that appeared on the ISU Integrated Crop Management website yesterday, explaining what corn growers need to know about this disease and urging farmers and agronomists to scout their corn fields in July and August—especially the fields that are most likely to be "at risk."
Those "at risk" fields are planted to corn hybrids that are susceptible to this disease. Also, "at risk" fields are where corn-on-corn is planted. If you find fields where this foliar leaf disease is prevalent, Robertson encourages you to act now and schedule a fungicide application for those fields. She notes that this disease this summer in Iowa is showing up a little earlier than usual.
So far, most of the reports of northern corn leaf blight in Iowa have come from central and western Iowa. But since the pathogen that causes this disease is spread by wind and rain, the disease could be more widespread in the state. You need to get out and scout and keep a watchful eye out for it in your fields, she says. Robertson offers the following observations, information and scouting guidelines.
What to look for when scouting fields for this disease
Typical symptoms of the disease are large (1- to 6-inch long) cigar-shaped lesions that are usually tan (See Figure 1). NCLB is sometimes misdiagnosed as Goss's wilt and leaf blight although there are certain characteristics of the lesions that enable the two diseases to be differentiated.
Large elliptical lesions are typical symptoms of Northern corn leaf blight.
Warm, humid conditions favor development of this disease
Infection of corn by the northern corn leaf blight fungus (Exserohilum turcicium) occurs when temperatures are warm (65 to 80F) and the free water is present on the leaves for 6 to 18 hours. "Considering the warm, humid conditions we have been experiencing it is no wonder NCLB is developing in fields. What is a little unusual is to hear reports of the disease occurring in Iowa prior to tasseling," says Robertson.
And there's more to managing northern corn leaf blight than just spraying a fungicide, she notes. You need to take an integrated approach to managing this disease.
Managing NCLB requires an integrated approach
Fungicides may be used to reduce yield losses due to NCLB. Numerous fungicide trials across the Midwest have found that products that contain a triazole are usually more effective than products that do not contain this chemistry. The Corn Disease Working Group, which includes corn pathologists from across the U.S., has published a fungicide efficacy table that summarizes efficacy data of the most common products.
Farmers who have NCLB in their field should wait to apply fungicides until tasseling, since applications between the V12 and VT growth stage of corn may cause arrested ear development. However, if the cool, wet weather continues, fungicides applications should not be applied too late. To optimize fungicide efficacy, Robertson advises farmers and other applicators to ensure good coverage of the corn canopy; pay attention to nozzle type, sprayer pressure and application volume.
How can you manage this disease in future years?
So, looking ahead, how can you manage this corn disease in future years? What can you do to try to avoid it? "For future corn crops in the affected fields, rotation to a non-host crop will reduce survival of the pathogen by allowing time for infested crop residue to break down," says Robertson. "Many corn hybrids carry resistance genes to NCLB, so your choice of hybrid can also lower risk of the disease."
There are many races of the pathogen, and specific resistance genes have been identified in corn hybrids that are effective against specific races. An article that discusses the prevalent races of NCLB in the U.S, resistant genes present in hybrids has been published by corn pathologists at DuPont-Pioneer. However, you need to understand that resistance to this disease is not complete; so smaller, yellowish lesions may be visible on corn hybrids— even those with resistance. "We do not yet know what race or races are responsible for the current outbreak," says Robertson.
Alison Robertson, Ph.D., is an ISU associate professor/extension field crops pathologist. She can be reached at [email protected] or 515-294-6708.