There has been a lot of interest in the use of fungicides to increase corn grain yields.

There are two schools of thought on this. The first is that the strobilurin group of fungicides has some properties that may increase yield without disease. These effects are:

• Increased CO2 uptake – more carbon to make starch with
• Less respiration – the use of starch
• Better nitrogen use – greener plants
• Higher rates of photosynthesis – or capture of sunlight.

The second school of thought is that fungicides will control fungal diseases (which is what the word fungicide means) that destroy leaf area. More corn acres and more corn-on-corn acres may lead to more leaf disease. Most of the leaf diseases overwinter on crop residue (exception is common and southern rust), and then infect the new leaves of the corn plant. The severity of most of the corn leaf diseases is correlated with the amount of corn residue on the soil surface. Therefore, reduced tillage corn-on-corn fields are – at least theoretically – more susceptible to leaf diseases.

Making fungicides pay. Iowa State University Extension personnel and Corn Soybean Initiative Partners conducted 34 field scale trials in 2007 to evaluate corn fungicides as a way to increase corn grain yield. Information on the level of leaf disease, stalk rot and crop rotation was also gathered with these field scale trials. Several of the trials were in northwest Iowa. Read more here.

The results showed a positive 3.3 bushel per acre yield increase. About three-fourths of the plots showed a positive yield response and about one-third of the plots showed a profitable response to the fungicide application.

The greatest yield responses came from areas that had the most leaf disease. Most of the plots did not have a great deal of leaf disease.

Using this data for 2008. The data from these ISU plots and others like it in the Midwest show that corn fungicides are the most beneficial when leaf disease is present. It appears that the corn fungicides – as a general statement – do not increase yields that much without the presence of leaf disease.

Therefore, a farmer can decide in season to use a fungicide based on the level of leaf disease that is present.

Fields that are the most likely to benefit from fungicide are:

• corn on corn fields
• fields with susceptible hybrids,
• Fields with a high yield potential.

Scouting for leaf disease. Check fields for leaf disease if the above criteria are present.
Hybrids that are susceptible (or moderately susceptible) to leaf diseases:

• A fungicide application may be warranted if disease is present on or above the third leaf below the ear leaf on 50 percent of the plants at tasseling stage. Hybrids that are intermediate in their susceptibility to leaf disease (applies to many hybrids).
• a fungicide need only be applied if conditions are favorable for disease development. Conditions that favor disease development occur when disease is present on or above the third leaf below the ear leaf or higher on 50 percent of the plants at tasseling time, the weather is warm and humid, and the field has a history of GLS.

Northwest Iowa. The trials that were conducted in northwest Iowa showed some response to corn fungicides –about a 4- 5 bu/a yield increase. However, there were very low levels of leaf disease present in the plots. Typically, northern Iowa has less corn disease than other parts of the state – because of rainfall occurrence, relative humidity levels and relatively low nighttime temperatures.

More Info. Daren Mueller, Iowa State University Extension Plant Pathologist has identified the following links for some more reading on the topic of fungicide use on corn.

• Foliar Fungicides for Corn and Soybean – Don’t Rush to Spray – University of Wisconsin
• Fungicide use in corn and soybean: to apply or not to apply? That is the question – University of Kentucky
• Some Fungicide Application Basics – how not to blow a great tool – Ohio State University
• Making Profitable Fungicide Applications in Corn – University of Illinois
• Should Hail-Damaged Corn be Treated With Foliar Fungicides? – University of Nebraska

Learn more about leaf diseases with corn on corn.

SOURCE: ISU’s Paul Kassel

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