Integrated Crop Management

Before applying fungicides to corn: Stop! Look! Consider!

There is considerable interest across Iowa and the whole Corn Belt in applying fungicides to field corn. In the past when corn prices were down below $2 a bushel, the decision to apply a fungicide was easy--no. This growing season, the high price of corn and increased disease risk due to increased corn-after-corn acreage has many producers considering fungicide applications as a means to increase yields.

Stop!

Before applying a fungicide to corn this season, it would be prudent to note the resistance to foliar disease of the hybrid being grown. The most common foliar diseases in Iowa are common rust, gray leaf spot, northern leaf blight, and, to a lesser extent, eyespot (see photos). Back in the mid-1990s, research done in southeast Iowa by Gary Munkvold, Iowa State University plant pathologist, and others demonstrated a single application of fungicide could be profitable; however, profitability was strongly influenced by gray leaf spot susceptibility. In other words, the chance of increased yield and making a profit only was likely on highly susceptible hybrids. On intermediate and moderately resistant hybrids, any yield benefit would not cover the costs of the fungicide application.

Gray leaf spot (early symptoms). (Alison Robertson) [1]Gray leaf spot (early symptoms). (Alison Robertson) Northern leaf blight. (Alison Robertson) [2]Northern leaf blight. (Alison Robertson) Eyespot. (Alison Robertson) [3]Eyespot. (Alison Robertson)

Look!

Many of the foliar diseases in Iowa start on the bottom leaves of the corn plant and gradually move up the plant depending on environmental conditions. Thus, scouting the field can give us a very good indication of the disease pressure in that field. The best time to start scouting is immediately prior to tasseling. Look for disease development on the lower leaves of the corn plant up to and including the ear leaf. Remember, it is the ear leaf and leaves above that contribute 75 to 90 percent of the carbohydrates to grain fill. Thus, these are the leaves we need to protect. If disease is not present on the leaves below the ear leaf, a fungicide application may not be warranted. Continue scouting on a weekly basis.

Consider!

Other factors to consider when deciding whether to spray a fungicide or not include:

Degrees of severity in the disease triangle. [6]Degrees of severity in the disease triangle.

There are numerous fungicides listed for use on corn in Iowa. Research has shown that those containing active ingredients belonging to the strobilurin and/or triazole groups are the most effective against foliar pathogens of corn (Table 1).

In conclusion, producers who are considering making fungicide applications for "plant health" benefits should be aware that a significant economic response is not assured. The jury is still out. The profitability of fungicide applications on corn is determined by numerous factors, so stop, look, and consider the above factors before you spray.

Table 1. Fungicides listed for use on corn in Iowa.

Adjuvants Coverage (gpa)
Fungicide Target DiseasesApplication Rate
(fl oz/acre)
Application
Timing
Ground Air Ground Air
Headline® Anthracnose
Common rust
Gray leaf spot
Northern leaf blight
Northern leaf spot
Physoderma brown spot
Southern leaf blight
Southern rust
Yellow leaf blight
6-12 Prior to disease development NIS @ 1 pt/100 gal COC @ 1 pt/acre 20 2-5
Quadris® Common rust
Eyespot
Gray leaf spot
Northern leaf blight
Northern leaf spot
Southern leaf blight
Southern rust
6-15.5 Prior to disease development NIS @ 1 pt/100 gal Sufficient
water volume
for adequate
coverage
5
Quilt® Common rust
Eyespot
Gray leaf spot
Northern leaf blight
Northern leaf spot
Southern leaf blight
Southern rust
7-14 At onset of disease
to brown silk
NIS @ 1 pt/100 gal NIS @ 1 qt/100 gal spray
COC @ 1 pt/acre
Sufficient
water volume
for adequate
coverage
2-5
Stratego® Common rust
Eyespot
Gray leaf spot
Northern leaf blight
Northern leaf spot
Southern leaf blight
Southern rust
7-12 At onset of disease
through to end of
silking (R2)
NIS @ 1 pt/100 gal COC @ 1 pt/acre 10 2-5
Tilt®, Propimax® Common rust
Eyespot
Gray leaf spot
Northern leaf blight
Northern leaf spot
Southern leaf blight
Southern rust
2-4 Apply when disease
first appears
NIS @ 1 pt/100 gal 10 5

Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases. Daren Mueller is an extension plant pathologist with the Iowa State University Corn and Soybean Initiative and the Pest Management and the Environment Program.

This article originally appeared on pages 201-203 of the IC-498(17) -- July 2, 2007 issue.


Source URL:
http://www.ipm.iastate.edu/ipm/icm//ipm/icm/2007/7-2/cornfungicides.html