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Thoughts on applying a fungicide at V5

By Alison Robertson
 
#plant19 has been a year to remember, and not necessarily in a good way. A couple of days ago we talked about anthracnose leaf blight on corn. Although we suggested there was little reason to fret, there have been some conversations around whether to apply a fungicide at growth stage V5, to mitigate the disease and perhaps stalk rot later on.  In addition, you’ll be spraying an herbicide, so why not throw a fungicide into the tank as well?
 
I’ll be the first to admit I am not a fan of applying fungicides at V5, and even more so with corn prices not to mention the additional issues we have seen with #plant19 (delayed planting, slow emergence, the cool start to the growing season, etc.) that will affect yield potential.
 
Here are some of my thoughts:
  • The first five leaves of a corn plant die and fall off the plant soon after canopy closure. They do not contribute to yield. These are the leaves which receive almost all of the V5 fungicide application.
  • Remember a fungicide is only active on the leaves to which it is applied. The fungicide may move through the leaf tissue to the margins of the leaf, but it will not translocate through the whole plant or to new leaves that emerge from the canopy. Also, a fungicide is usually only effective for 14-21 days, although some of the newer products may last longer.
  • Fungicides are used to manage foliar diseases. It is extremely unusual to see foliar disease on these first five leaves of corn in Iowa.  In corn-on-corn fields, anthracnose leaf blight may be present.  In my experience, this disease rarely moves up out of the lower canopy.  There have been suggestions that anthracnose leaf blight can lead to stalk rot later in the season.  Research done at ISU and Wisconsin found no relationship between anthracnose leaf blight and stalk rot.  This is probably not surprising since resistance to the leaf blight and stalk rot phases of anthracnose are controlled by separate genetic mechanisms. 
  • Some farmers use fungicides for physiological benefits that translate to yield benefits.  I have measured yield increases in my fungicide trials in Iowa since 2010. The yield benefit associated with a V5 application ranged from -2.5 bu/a to 6.0 bu/A (average 2.1 bu/A), compared to 1.2 bu/A to 17.7 bu/A (average 6.0 bu/A) with an application at tasseling. Consequently, there is little chance of a return on investment (ROI) with an application of fungicide at V5, unless grain prices bounce back to $7/bu.
  • An application of a fungicide at V5 will not negate the application of a fungicide later in the season.  Most corn diseases in Iowa start to show up in July as the corn approaches tasseling. Thus, if the weather is conducive for disease (gray leaf spot, northern corn leaf blight, tar spot, etc.) development, and particularly if a hybrid is susceptible to foliar disease, an application of a fungicide around tasseling may be necessary – with a better chance for a ROI.