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Issue No. 16, Article 4/July 10, 2009

Considerations for Applying Foliar Fungicides to Corn

Applications of foliar fungicides to corn in Illinois and other midwestern states have been on the rise in the last two seasons, and this trend likely will continue in 2009. Summaries of results from university corn fungicide trials conducted in 2007 and 2008 in multiple states and Ontario, Canada, indicate that foliar fungicides did not provide an economic benefit every time they were applied. In fact, the range of yield responses was very wide, from approximately -20 bushels per acre to more than 20 bushels per acre relative to untreated checks, and the average yield responses were 3 and 3.6 bushels per acre in 2007 and 2008, respectively. A yield response in the vicinity of 3 bushels per acre will not be enough to cover the cost of the fungicide application unless the corn price is well over $6 a bushel (Table 1).

 


Summary of results from university corn fungicide trials conducted in 2007 and 2008 in multiple states and Ontario, Canada (2007 summary compiled by Carl Bradley; 2008 summary compiled by Greg Shaner, Purdue University).

So how should these university summaries be interpreted? The primary interpretation that can be made is that the yield response of corn to a foliar fungicide is highly variable when disease control is not specifically targeted. In other words, the foliar fungicide applications made in these research trials were applied to corn based on the growth stage of corn only, and no considerations of disease pressure were made.

Dr. Greg Shaner, Purdue University, compiled results of 2008 university research trials based on disease pressure levels. He found that when disease severity was less than 5%, the average yield response was 1.2 bushels per acre; however, when disease severity was 5% or greater, the average yield response was 7.5 bushels per acre.

In Illinois, corn fungicide trials were conducted at eight locations. At five locations, the yield responses to foliar fungicides were low (3 bushels per acre or less). Disease pressure was also low (12% severity or less). At the other three locations, the yield responses were high enough for the fungicide applications to be profitable (11, 19, and 26 bushels per acre), and disease pressure was moderate to high (19%, 34%, and 42%). The bottom line of these research results is that the use of foliar fungicides was profitable only when diseases were present at a high enough level to cause economic yield losses.


Yield response to a fungicide based on disease pressure (from 2008 data summary compiled by Greg Shaner, Purdue University).


Effect of fungicides on corn yield at eight locations in Illinois in 2008. The biggest yield responses were observed at locations with moderate to high disease pressure.

Foliar fungicides for disease control: A novel idea?

The information just described is very good evidence that foliar fungicide applications provide profitable yield benefits on a consistent basis only when disease pressure is severe enough to cause economic losses. The idea that fungicides can protect against yield losses when disease pressure is severe is certainly not new. However, it has become evident that the issue of severity level is not stressed clearly enough in fungicide product advertisements. Claims of yield enhancement, improved growth efficiency, and stress tolerance are often the messages being touted, with little to no mention of diseases.

A supplemental label for Headline fungicide submitted by BASF was recently approved by the US EPA. This "Plant Health" supplemental label makes claims of improved growth efficiency (improved plant utilization of nitrogen) and stress tolerance (drought, heat, cold temperatures, and ozone damage). It is true that strobilurin fungicides can have other impacts on plants besides disease control. However, these "other effects" do not automatically translate into yield increases. Despite supplemental labels and advertisements, the primary reason to apply a foliar fungicide to corn should be to protect against disease.

Determining a corn field's risk of developing a severe foliar disease problem can help with making a fungicide application decision. Certain production practices and factors can play a role in a field's foliar disease risk. The likelihood of greater disease pressure increases when many of these factors are present:

Previous crop and tillage practice. When corn was the previous crop and substantial residue is left on the soil surface, the risk of foliar diseases increases. Many of the fungal pathogens that cause foliar diseases on corn survive in corn debris.

Planting date. Research conducted in Illinois has indicated that late-planted corn is more at risk for some foliar diseases than early-planted corn.

Hybrid susceptibility. Most hybrids are rated for their susceptibility to diseases like gray leaf spot and northern leaf blight. When hybrids with greater susceptibility to these diseases are planted, the risk of the diseases increases. University trials conducted in 2007 indicated that hybrids with a "fair to poor" rating for gray leaf spot resistance had a yield response of 6 bushels per acre to a foliar fungicide, compared to a 4 bushels per acre when a foliar fungicide was applied to hybrids with a "good to excellent" rating for gray leaf spot resistance.


Results from multi-university trials in 2007 showing that hybrids with less disease resistance had a better yield response to foliar fungicides.

Weather and environment. High relative humidity and moisture are important to the development of foliar disease on corn. Having leaves wet longer is favorable for infection by fungal pathogens.

Disease observations. Scouting fields prior to tassel emergence may give an indication of potential disease pressure. The earlier that some diseases are apparent, the greater the risk of losing yield. No hard-and-fast economic thresholds are available for foliar corn diseases, but scouting can help suggest how quickly diseases are building up on the lower leaves. Be aware of the following fungicide guidelines based on scouting observations:

  • For susceptible or moderately susceptible hybrids, consider a fungicide application if the disease is present on the third leaf below the ear or higher on 50% of the plants before tasseling.
  • For intermediate hybrids, consider a fungicide application if conditions and factors are favorable for disease and if disease is present on the third leaf below the ear or higher on 50% of the plants before tasseling.
  • For resistant hybrids, a fungicide application generally is not recommended, but field scouting is still important.

Which product(s) should I use?

In University of Illinois research trials, Headline, Quadris, Quilt, and Stratego have been tested in multiple years and locations. The results have indicated that when diseases are at a level high enough to reduce yield, all of these products generally are statistically equal in their effectiveness for disease control. Table 2 compares products based on their active ingredients. When reviewing the table, it is important to be aware that an effective rate of one strobilurin active ingredient may not be the same as another strobilurin active ingredient (i.e. it may be like comparing apples to oranges).

The bottom line is that when disease pressure is high enough to reduce yields, most of the fungicide products available for corn will do a good job of protecting against diseases and yield losses.--Carl A. Bradley

Author:
Carl A. Bradley

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