Late-Season NCLB: How to Minimize Yield Loss

The sprayer or the combine, that is the question, at least when it comes to contemplating management strategies for late-season northern corn leaf blight (NCLB). And it’s among the most common foliar diseases corn farmers face each season, with symptoms often intensifying in mid-to-late summer as humidity and residue-borne pathogens create ideal conditions for infection. While fungicide programs applied at growth stages VT to R1 remain the primary tool for protecting yield from NCLB, once corn has reached the R5 or R6 stage by late August or September, many fields have matured past the window where another spray delivers strong ROI. At this stage, the most valuable conversations you can have with your customers are not about chemistry, but about scouting, scoring and prioritizing fields for harvest. With that in mind, let’s first review the basics of NCLB and how it can affect stalk quality and harvest timing.

Background

NCLB is caused by the fungus Exserohilum turcicum. Present in the U.S. since the late 1800s, its prevalence and severity have increased since the early 2000s. This is often attributed to wider adoption of no-till and reduced-till farming, shifts in both hybrid genetics and pathogen races, plus the increase of favorable humid, temperate summers. Today, it ranks as one of the most damaging crop diseases, capable of causing 30% to 50% yield loss in severe cases.¹

Identification

Early signs of NCLB infection appear as oblong, narrow, tan lesions that run parallel to the leaf edge. As the disease progresses, these lesions become larger. In situations of high humidity, lesions form black or olive-colored fungal spores that can look gray or “dirty” to the naked eye. A single lesion can measure anywhere from one inch to seven inches in length, and lesions may grow together over time, creating large areas of dead tissue. It’s important to note that these symptoms may resemble southern corn leaf blight or Goss’s wilt, so accurate identification is key.

Photograph of leaf with NCLB lesions.

Disease cycle

NCLB begins as a fungus that overwinters as mycelia and conidia in crop residue such as corn leaves and husks. When the environment becomes suitable in spring and early summer, spores grow on this infected crop residue. The spores are then spread to new plants via wind or rain splash. Infection occurs when water is present on the leaf surface for 6-18 hours with temperatures between 65°F and 80°F. While infection often begins low on the plant and travels upward, severe infections may appear on the upper canopy as a result of spores spread by wind from southern areas, such as Mexico or the Caribbean. NCLB is more likely to spread when temperatures are moderate with high humidity. Frequent light showers and heavy dews are also known to encourage the growth of NCLB.

NCLB life cycle.

Management

NCLB does respond to several management methods including planting resistant hybrids, crop rotation, tillage and fungicides. However, most of these methods are not an option late in the growing season, including an additional spray pass. Dennis Holland, Product and Sales Agronomist, Corteva Agriscience, explains, “Fungicide applications are most effective early in the season when the disease is active. However, in late summer, a fungicide pass won’t provide a good ROI, because fungicides protect healthy leaf area and slow new disease development — they do not restore tissue already killed by NCLB. At that stage, it’s time to think about getting the most affected plants harvested first.”

Harvest order: a good late-season approach

When late-season NCLB infects corn at or after the R5-R6 growth stage, building a harvest priority schedule is critical. Dennis says that’s because, “NCLB infections will rob the plant of photosynthetic area causing weaker stalks. Grain quality can also be reduced, and the cobs can be soft.” These issues increase the risk of stalk rot and lodging, which can make for a slow, difficult and frustrating harvest while reducing yield.

Building a harvest priority schedule

The steps below outline how to assess NCLB severity, lodging risk and field conditions so farmers can map out a sequence for which fields to harvest first.

Scout and score fields for risk through September
- Walk fields and do a pinch/push test on multiple plants.
- Flag fields as high risk for lodging where >10%-15% of stalks fail the pinch/push test.
- Note the severity of leaf blight, as it can indicate the level of stalk stress, especially if blight is present on the ear leaf and higher on the plant.
Determine harvest order based on standability and disease, prioritizing fields based on:
- Stalk strength – Fields flagged as high risk in the pinch/push test.
- Disease pressure – Fields with extensive leaf blight.
- Maturity and moisture – Plants with substantial NCLB infestation may mature and dry faster than they would normally. Earlier harvesting may be required to avoid missing target moisture stages of heavily infected fields.
Harvest fields with the most diseased, weakest stalk plants first to reduce losses.

Once NCLB has taken hold and corn is well into grain fill, the most profitable management technique is likely not another spray pass, but a prioritized harvest plan. By scouting, scoring and prioritizing fields with the weakest plants for early harvest, you can help your customers protect yield that is already in the canopy, cut lodging losses and maintain grain quality.

¹ Tamra A. Jackson-Ziems, “Northern Corn Leaf Blight,” NebGuide, December 2015, https://extensionpubs.unl.edu/publication/g2270/2016/pdf/view/g2270-2016.pdf.
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