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Resistant Hybrids, Residue Management, and Fungicides Can Fight Northern Corn Leaf Blight
If 2016 is wet and cool like 2014 and 2015, you’ll probably see lots of northern corn leaf blight (NCLB) in your corn.
Large elliptical-shape lesions on corn leaves herald this fungal disease. NCLB can slice corn yields up to 30% if lesions are present prior to or at tasseling, according to Purdue University plant pathologists.
NCLB survives in residue and surfaces during cool and wet conditions. “For infection to occur, corn leaves have to be wet at least six hours long,” says Alison Robertson, Iowa State University (ISU) Extension plant pathologist. Purdue University literature indicates NCLB infections thrive between temperatures of 64°F. and 81°F.
Sunshine helps curb NCLB. “When you have bright sun, spores (that spread NCLB) are not as viable,” she says.
Ditto for dry weather. It stopped NCLB in its tracks in Iowa during July 2014.
“It was dry,” says Robertson. “July was cooler than normal, but there was not as much moisture (as before) and not as much inoculum produced. So, the disease just stopped.”
In August 2014, though, rainfall fueled its advance. “The longer that leaves are wet, the more chance there will be of spores germinating and infecting and causing disease,” says Robertson.
In some cases, farmers who sprayed at R1 (early silk that is the recommended time in ISU tests) had to spray again at R5 (dent) to treat for NCLB in 2014.
“Fourteen to 21 days later (after initial application), if disease pressure remains high, and if conditions are conducive to disease, another spray treatment can be beneficial,” says Randy Myers, Bayer CropScience product development manager. “Even at R5, disease can impact corn yields.”
So how do you manage NCLB?
“Resistant hybrids are the best way,” says Robertson. “Most (seed) companies score for NCLB.”
Rotating away from corn to reduce residue is another way, as corn residue harbors the NCLB fungus. In no-till or reduced-till fields with a history of NCLB, a two-year rotation out of corn may be needed to reduce the amount of disease in the following corn crop, according to Purdue plant pathologists.
Fungicides are another tool. ISU tests have shown positive results when fungicides were applied, particularly in fields under heavy NCLB infestations.
ISU trials show the best time to apply fungicides on corn is at the R1 (early silking) stage, she says. ISU tests show that no one fungicide in 2015 trials stood out above others.
“Basically, all fungicides worked well at controlling NCLB,” she says. “If you spray at R1, you will get good disease control.”
There has been a move by some companies to also spray fungicide early at the V5 (five leaves with visible leaf collars) stage. ISU tests, though, have shown NCLB severity to be reduced the most with R1 applications.
“With northern corn leaf blight, you cannot get away with just a V5 application,” says Robertson. “V5 and R1 applications may be prudent when very susceptible hybrids are planted. But that is only on very susceptible hybrids only.”
Foliar diseases also can spur the development of stalk rots later in the season. This can cause problems with corn toppling over prior to harvest, as happened in some fields in 2015.
Fungicides applied at R1 can help curb this, but it’s due to the fungicide-curbing foliar diseases and not stalk rots. “No fungicide directly affects the stalk-rot pathogen,” she says.
No thresholds exist for NCLB. However, scouting fields around the V14 phase (just prior to tasseling) can help to determine field-disease pressure.
Kiersten Wise, Purdue University Extension plant pathologist, wrote in a summer 2015 Purdue Crop and Pest newsletter that 50% of plants in a field need to have disease lesions present. However, additional factors must also be present like prolific residue and cool temperatures and humid conditions.
Fungicides can pay off, but Wise cautions that crop conditions in Indiana vary widely. Thus, you should think carefully before making decisions to add inputs into your crop. She advises considering expected crop price, crop condition, planting date, predicted weather conditions, and the impact these factors may have on your economic return this year.
Fungicide recommendations developed by the national Corn Disease Working Group can be found at http://www.extension.purdue.edu/extmedia/BP/BP-160-W.pdf. For more information on northern corn leaf blight, please read Purdue Extension bulletin BP-084-W.