By Alison Robertson, Ph.D., Plant Pathology and Microbiology
Symptoms of NCLB
Typical symptoms of the disease are large (1- to 6-inch long) cigar shaped lesions that are usually tan (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.
Photo. Large elliptical lesions of Northern corn leaf blight.
Warm, humid conditions favor NCLB
Infection of corn by the NCLB 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.
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 are more effective than those 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 that have NCLB in their field should wait to apply fungicides until tasseling since applications between V12 and VT may cause arrested ear development, but if the cool, wet weather continues, fungicides applications should not be applied too late. To optimize fungicide efficacy, ensure good coverage of the canopy; pay attention to nozzle type, sprayer pressure and application volume.
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 residue to break down. Many hybrids carry resistance genes to NCLB, so 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 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. Resistance is not complete, so smaller, yellowish lesions may be visible on hybrids with resistance. We do not yet know what race(s) is responsible for the current outbreak.
Agronomist and farmers are encouraged to scout at risk fields (susceptible hybrids, corn-on-corn fields) for NCLB and schedule a fungicide application for fields were the disease is prevalent.
Alison Robertson, Ph.D., is an associate professor/extension field crops pathologist. She can be reached at firstname.lastname@example.org or 515-294-6708.