July 9, 2015
Scout Corn Fields NOW for Northern Corn Leaf Blight
I'm seeing and hearing more reports of northern corn leaf blight (NCLB) this year. While far from an epidemic, enough fields are showing at least a few lesions that it would be a good idea for farmers to get out and see what is happening in their fields, then plan their management accordingly.
Early NCLB symptoms appear as long, narrow, tan lesions that form parallel to leaf margins. As these lesions develop, the typical symptoms of NCLB will be observed: long, oblong, or "cigar-shaped" tan or grayish lesions. The lesions produce olive-green or black fungal spores when humidity is high, which can give the lesions a dark or dirty appearance.
What makes NCLB more of a concern this year is the time when it is appearing in fields. This is a little earlier than normal for the disease to develop and since many fields were planted a little later than normal, this creates a situation with potential for the disease to cause more yield losses.
I want to be clear, that is NOT a blanket recommendation to treat all fields with a fungicide to reduce potential losses... you need to scout and evaluate your fields first. There can be night and day differences on disease development between adjacent fields, or even within the same field if different hybrids were planted. If you haven't already, go back to your seed book or talk to your seed dealer and determine which corn hybrids have the poorest resistance to northern corn leaf blight. Then scout those fields first.
No treatment thresholds have been established for NCLB. But, you can judge your risk for the disease developing to levels where it limits yields by considering the factors that put your field more at risk. The more of these high risk factors you have in your field, the greater the chance of NCLB lowering your yields and the greater the likelihood of getting a return on the cost of a fungicide application. High risk factors include:
–¢ Poor hybrid disease rating(s) for NCLB
–¢ Early disease development, especially during pre-tassel growth stages
–¢ Continuous corn
–¢ Fields with a history of severe NCLB
–¢ Substantial corn residue
–¢ Weather forecast for humid/wet weather and moderate temperatures
The development and spread of NCLB prior to tasseling could substantially reduce corn yield, particularly as lesions develop and expand, killing leaf area that's necessary for grain fill later. It is most important to protect leaves at the ear leaf and above because they contribute the most to grain fill.
In some fields, the disease has already reached leaves 8-9. With the early development of NCLB now, it might be necessary to make a foliar fungicide application to slow disease spread in susceptible hybrids and protect uninfected leaf tissue.
Lesions appear several days after infection occurs. So it is possible that one to two leaves are already infected above the highest leaf on the plant that has recognizable lesions. To determine if a fungicide application is economical for you, consider:
–¢ corn price,
–¢ yield potential,
–¢ cost of treatment, and
–¢ disease severity.
For more information on NCLB, check out http://cropwatch.unl.edu/ for several good articles and links or contact your local Nebraska Extension office.