Is a Fungicide Application to Corn at V5 a Worthwhile Input?

May 31, 2023

By Alison Robertson

It’s a great time of year!  The weather is finally warming up enough to wear shorts and green stripes are finally occurring across the landscape as corn (and beans) emerge.  USDA reports 85% of the corn crop had emerged for the week ending May 30.

Some early planted corn is at V5 and it won’t be long before the rest of the crop reaches V5.  Post emergent herbicide applications will be on everyone’s minds, so why not throw a fungicide into the tank as well?

 It’s no secret that I am not a fan of applying fungicides at V5. Below I reiterate some thoughts from a previous blog.

  • 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.  
  • Since the first five leaves of a corn plant are fully expanded, they receive almost all the V5 fungicide application. These treated leaves die and fall off the plant soon after canopy closure (around V11, which is approximately 2 weeks after V5). These leaves do not contribute to yield.
  • Depending on the product applied, a fungicide is effective for 14 days to 42 days (older to newer products, respectively).
  • 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 continous corn fields, anthracnose leaf blight may become present IF we get some rain in the next couple of weeks.  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 in Iowa 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. 
  • Everyone is concerned about tar spot. Thus far, all research across the Midwest has found that a V5 application has little effect against tar spot (probably because if the disease occurs early, it’s more likely to be first observed on the 8th to 10th leaves, and these leaves are not protected by the V5 application of fungicide.)
  • Some farmers use fungicides for physiological benefits that translate to yield benefits.  I measured yield increases in my fungicide trials in Iowa from 2010 through 2017. 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) benefit 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 the need for an application of 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. Regarding tar spot, data from fungicide trials across the Midwest have consistently shown an application of fungicide at R1 is best for tar spot management in most years.  In wet growing seasons when the disease is observed prior to VT, two applications may be needed.
  • Lastly, a V5 application of fungicide is effective against Physoderma brown spot but not Physoderma node rot. 
  • We have fungicide trials at three locations in Iowa this year to evaluate the potential of a V5 application to reduce crown rot.  Hopefully we get crown rot this year so that we’ll have data to share in the future.  But for the moment, in my opinion, the jury’s still out on V5 fungicide applications and crown rot.
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