Agronomy •  2019-04-24

Are Corn Fungicides Worth the Additional Cost?

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by Bill Mahanna and Ev Thomas

July 2012

In recent years, there has been an explosion of interest in the use of foliar (leaf) fungicide treatments. That extra attention has paralleled the steady upward climb in grain prices. Industry estimates are that about 13% of corn acres and 12% of soybean acres were treated with fungicides two years ago.

Fungicides have various modes of action including electron blockers within the mitochondria or specific enzyme blockers which limit the fungal ability to metabolize nutrients to fuel their growth. The desired outcome is improved plant health resulting in higher grain (starch) yields which is desirable whether the crop will be harvested for grain or silage. Corn fungicides are applied to inhibit foliar fungal infections such as Gray Leaf Spot, Northern and Southern Leaf Blight, Common and Southern Rust, Anthracnose, and Eyespot.

Conflicting research

There are strong opinions on both sides of the fungicide debate. Recommendations from the University of Wisconsin suggest that there is not a consistent economic return from fungicide usage and that growers should focus primarily on hybrid resistance to foliar diseases. Contrast that with research from the University of Illinois that showed an average 7.6 bushel per acre yield advantage to fungicide treatment over the past three growing seasons and when the crop was under high disease pressure, anywhere from a 15 to 20 bushel per acre yield response.

Seed companies have also conducted research on fungicides. On-farm trials with 475 growers conducted from 2007 to 2011 by Pioneer Hi-Bred showed an average 7.0 bushels per acre response to foliar fungicide application. Consistent with the Illinois research, 10 small-plot research locations harvested in 2009 by Pioneer showed fungicide yield responses varied from 0.6 bushel to 22.6 bushels per acre, depending on disease pressure and hybrid susceptibility.

There does seem to be agreement as to conditions which favor foliar fungicide applications. These include:

  • Planting hybrids susceptible to foliar diseases
  • Fields with high residue, such as corn-following-corn, and no-till or strip-till
  • Extended warm, wet, humid growing conditions
  • Planting at very high populations

While no research data exists on narrow-row silage corn (15-inch rows), this may also contribute to a high-humidity, micro-environment more conducive to foliar diseases. Later-planted fields and/or later-maturing hybrids may also respond better to fungicide treatments because they are in the important grain filling period as foliar disease development peaks in late summer.

Results of a three-year joint research study by the University of Tennessee and Pioneer Hi-Bred further reinforces the need to focus on hybrid selection. The study showed that the probability of a positive economic return from using a fungicide was directly related to the susceptibility of a hybrid to the predominant leaf diseases. It is clear that corn grain and corn silage growers should fine-tune their hybrid selection process by assessing hybrid disease ratings for foliar diseases.

Simple economics would suggest that an investment of about $30 per acre in fungicide (and application) would need to generate five more bushels of $6 corn to break-even. For corn silage valued at $40 per ton (35% dry matter), a yield gain of 0.75 tons as feed or 0.35 tons of silage dry matter would be needed to break-even. As the value of the grain (starch) goes up and agronomic practices such as reduced tillage and high plant populations continue to predominate, more growers are viewing fungicides as a defensive or insurance-type tool.

It still takes scouting

The use of fungicides does not eliminate the need for spending time walking the crop and scouting for diseases. This is especially true if considering early-season fungicide applications (prior to the fifth leaf collar stage) which can be attractive by eliminating one application expense when mixing fungicide with a postemergence herbicide. Unless scouting reveals early fungal infections, it may be best to wait until more leaf area is exposed. This is especially true given that most foliar diseases don't become entrenched until later in the growing season (after pollination) and that the average residual period is typically only 7 to 21 days.

Plant pathologists suggest that fungicide application should be considered if infection has moved up through the leaf canopy. When scouting plants, concern should mount when more than 5% of the ear leaf area contains lesions by tasseling and silking (VT-R1). This level of infection, at such an early growth stage, will likely expand in severity and reach the economic threshold of over 15% leaf area infection. As a result, the plant's ability to deposit starch throughout the reproductive stages will decline significantly.

What about corn silage?

There have been limited fungicide studies with corn silage, although benefits derived for grain corn should similarly benefit silage growers. A 2007 field trial conducted by University of Wisconsin Extension specialists showed that fungicide treatment resulted in a 0.7 ton gain in silage dry matter yield and a 1.9% boost in starch content. While these improvements were not statistically significant, they do appear to be biologically and economically encouraging.

This study also found no significant reduction in mold, yeast, and mycotoxin levels, but it should be noted that even the untreated plots were essentially devoid of mold contamination. The ability of fungicides to reduce mold needs further research as they would not prevent spores infecting the ear by entering silk channels during pollination.

Silage growers have also questioned if fungicide application would have any negative effect on the anaerobic bacteria plant populations. Anaerobic bacteria are responsible for fermentation in silages that are not inoculated with a commercial product. Technical experts at three fungicide manufacturers confirm that the mode of action of fungicides have no detrimental effect on these anaerobes responsible for fermentation.

Practical issues to discuss with your agronomist or chemical sales professional when selecting a fungicide include:

  • Disease threshold considerations
  • Timing of application(s)
  • Aerial application limitations
  • Residual activity
  • Curative properties
  • Plant response issues
  • Redistribution capability to ensure coverage deep in the leaf canopy

Do not expect fungicides to always return a profit, nor to necessarily reduce mold and mycotoxin problems. However, there is data suggesting that fungicides can be a very effective tool for managing foliar diseases and deliver healthier plants with higher grain (starch) content. Modern fungicides should certainly be considered as a viable management tool, especially in challenging, high-yield environments with hybrids susceptible to foliar disease.


Originally published in Hoard's Dairyman, July 2012. Reproduced with permission.