The corn fungicide dilemma: when should a fungicide be applied? Part I of V, yield in the absence of measureable disease

To keep each of the sections short I’ve separated the entire segment regarding fungicide application in corn into 5 general segments.

2011 marks my 5^th season with Mississippi State University.  Since I started this job on May 15 2007, one particular question has been asked more than any other question: should I apply a fungicide to corn at tassel?  I’m not the only one that can make this claim regarding this particular question.  Every single plant pathologist throughout the corn belt is asked this particular question between the winter meetings and early to mid-July depending on geographic location.  Additionally, this continues to be a contentious issue between chemical companies and members of the academic community for numerous reasons.  At present, fungicides are being promoted in corn production from V5 through VT/R1 as a “plant health” application regardless of whether or not disease is present.  While no data is present from unbiased university trials conducted by plant pathologists regarding V5 fungicide applications in the southern U.S. (to my knowledge) the greater northern data set suggests that applications made at early vegetative stages are not beneficial (i.e. translating into yield) and the results are sporadic at best.  Fungicides have also been suggested in rare situations to benefit corn by reducing the potential of drought stress.  Even though this specific situation has not been completely investigated in MS, data from the three year study conducted between 2007 and 2009 did include both irrigated and non-irrigated locations (stay tuned for Part II).  In addition, fungicides have been promoted as a “curative” application to hail damaged corn.  Be mindful, very little data exists to corroborate this suggestion and the single research article that is available suggests that fungicide applications following simulated hail (hail damage was simulated with a string mower since locating truly hail damaged plots is a difficult endeavor) are not beneficial in the long run (see Bradley and Ames, Plant Disease 94:83-86). 

Currently, foliar diseases are not an issue in the corn crop so applying a fungicide to prevent yield loss from a particular foliar disease would be a moot point at this juncture.  Moreover, the residual from any fungicide product will likely only result in a 14-28 day period where disease would likely be prevented.  Length of residual will depend on the environment, presence of a particular disease, overall disease pressure, particular product applied, and product rate applied.  Season long protection from a foliar fungicide application at V5 would not likely occur since most foliar diseases tend to occur following tasseling.  Moreover, the single most important delimiting factor in disease incidence will be the environment.  Remember, most of the corn hybrids we plant in our corn production system have fairly good disease packages with regards to diseases such as gray leaf spot, northern corn leaf blight, and southern corn leaf blight.  Common rust is typically not a yield limiting disease in our production system, eyespot is generally not observed, and anthracnose can be detected but has not occurred to levels where yield was jeopardized, at least in the 5 years that I’ve been working in MS.  Hybrids with resistance to southern corn rust are not typically planted in MS and there is only one hybrid that is commercially available with tolerance to this particular disease to my knowledge (Pioneer 33M52).

From 2007 to 2009 many individuals within the MSU system were involved in conducting fungicide trials in corn.  The major research endeavor was funded by the Mississippi Corn Promotion Board (2007-2009).  A total of 27 locations were assessed that included large plot on-farm locations, smaller experiment station locations, irrigated, non-irrigated, corn following corn situations, products applied by air, and products applied by ground equipment.  While yield was the major variable considered in all of the trials, all locations were rated for the presence of foliar diseases, percent chlorophyll post-treatment (measured with a SPAD meter from all of the locations, Part III of this series) and standability of corn plots post-application as measured by percent lodging (Part IV will discuss this aspect).  Fungicides were applied at all locations at tassel (VT) at the full label rate and included crop oil concentrate at a rate of 1.0% v/v.  Foliar diseases (common rust, northern corn leaf blight, southern corn leaf blight, and southern corn rust) were rated post-application and ranged from 0 to 3% regardless of year or particular disease on a whole plot basis.  I’ve calculated the response in each of the attached figures at a $4/bu (≥ 5 bu/A would be considered to be an economic benefit) and $6/bu (≥ 3.5 bu/A would be considered to be an economic benefit) selling price.  Four figures are included: Figure 1 presents the average of all locations conducted for each treatment, Figure 2 through Figure 4 presents each of the locations treated as an accordion figure for each of the respective fungicides used.  Data in Figures 2 through 4 are presented as the overall yield from each location, based on the average of 4 replicate plots, as compared to the untreated check.  A “break even” situation would result in a response where only the cost of the application and product would be recovered.  In all cases, a positive economic response occurred: 1 time in our data set following a Headline application (@ a $6/bu selling price), 1 or 7 times from a Propimax application based on a $4 or $6 selling price, respectively, and 3 or 7 times from a Quilt application based on a $4 or $6 selling price, respectively.  Essentially, this translates into a positive economic response resulting from an at tassel fungicide application in the literal absence of disease occurring less than 20% of the time with today’s commodity prices (based on 80 fungicide locations (27 for Headline, 26 for Quilt, 27 for Propimax) and 15 of those resulting in a positive economic return above the “break even” point).  Even though $6/bu corn increases the likelihood of an at tassel fungicide application paying off the results from the MS data set still suggest that the likelihood is sporadic at best.    

Following the data collection with regards to yield, and the low level of disease present at each of the locations utilized for the 3 year study, our suggestion to producers regarding the timed application of a fungicide in corn continues to be: scout for the presence of a yield-limiting disease and apply a fungicide to benefit in yield loss prevention rather than making a timed application at tassel.

I’ll close with a few statements regarding the development of fungicide resistance.  Even though resistance has not been reported to have developed in corn fungal populations the potential for fungicide resistance to develop at some point in the future is a real one.  I recently read an article where the individual interviewed essentially stated that “resistance would not develop with the application of full fungicide rates.”  Frankly, that statement couldn’t be further from the truth.  Within any fungal population there are individual spores that would be naturally tolerant/resistant to the use of a particular fungicide.  One difficult thing to consider is the sheer number of spores that are developed on a per acre basis.  Think along the lines of billions of spores.  To put that into perspective that would be 4,000 times more than the average pigweed if it produced 250,000 seeds.  Keep in mind, the pathogenic fungi infecting corn are microscopic and this would result in the production of spores by each of the colonies present across the entire acre and not just from a single colony.  Protecting fungicide technology is an important endeavor since the pipeline is not full of new products or active ingredients at this point.  In Part V I’ll share data from a heavy disease pressure situation during 2010 when southern corn rust was the predominant corn disease in the field.