In February, we wrote a Kentucky Pest News article on the supplementary label for Headline® fungicide which permitted the use of this pesticide for growth promotion and stress tolerance (http://www.uky.edu/Ag/kpn/kpn_09/pn_090224.html#Corn).  While Headline® and related strobilurin fungicides are excellent for control of certain diseases, the claims made go well beyond disease control to include enhanced growth efficiency, improved stress tolerance, improved tolerance to frost and hail, and so on.  The past article presented our concerns about these uses.  Last month, the American Phytopathological Society held a symposium entitled, “The Use of Fungicides to Promote Plant Physiological Benefits in Crops”.  Scientists from land grant universities, government and industry participated in this symposium, and everyone had a chance to see relevant data from a variety of sources, as well as to participate in the discussion. This article presents a capsule of the updated situation as we understand it.  We focus this article on Headline® because, to date, this is the only fungicide with a supplemental label which promotes uses in corn and soybean beyond disease control.

Corn
A thorough review of all available, validly conducted field research shows that there are numerous instances where Headline® and other strobilurin fungicides enhance yield in the absence of significant disease pressure.  Indeed, it is impressive to see yield increases in corn as high as 36 bu/acre from a single application of Headline®, in fields with very little disease pressure.  On the other hand, yield loss as great as -21 bu/acre from Headline® have been recorded as well.  Overall, the best available data indicate that a Headline® application met or exceeded a breakeven point of 7 bu/acre in 45% of the 203 trials examined, in the absence of significant disease pressure.  (With corn prices dropping and the current high cost of fungicide application, a breakeven point above 7 bu/acre is probably justifiable, but to be conservative, we’ll keep it at the 7 bu figure discussed at the symposium.)

A 45% breakeven scenario in the absence of significant disease pressure is pretty impressive, if you consider that we’re talking about a fungicide.  However, 45% is still not even equal to a coin toss.  In other words, in the absence of significant disease pressure, application of Headline® to corn is not likely to pay for itself. 

One of the conclusions from university data for corn is that this “yield bump” in the absence of significant disease is unpredictable.  There are no convincing data that give guidance to a producer, so that s/he knows when they can expect the yield bump.  University research shows clearly that some corn hybrids get a greater yield bump than others.  However, university studies also show that the effect is inconsistent from one environment to another.  In other words, just because you got the yield bump in Hybrid X in one field in one year, you are not necessarily going to see it in another.

The claims of improved stalk health are valid to a degree.  Improved stalk health certainly can result from the application of Headline®.  However, in most university trials where this effect is seen, it relates to control of foliar diseases.  In a small number of trials, improved stalk health was reported in the absence of foliar disease.  In these cases, it might relate to the “greening effect” extending the presence of green leaf tissue.  However, overall, most cases of improved stalk health relate to foliar disease control.  This is important because it gives guidance about how to best make the decision to apply the fungicide, as we will summarize at the end of this article.

The claims of improved tolerance to hail damage are exciting but aren’t necessarily based on valid research.  The only studies we are aware of that properly test these claims have not shown any benefit of Headline® in helping corn tolerate hail damage.

Soybean
Most interest surrounding the use of fungicides in soybean involves application of strobilurins, like Headline and Quadris®, or strobilurin + triazole premixes, such as Quilt® and Stratego®.  In 2005 and 2006, data from university trials were summarized for different fungicides, including Headline®, applied at early pod development.  In the absence of significant disease, a statistically significant yield increase occurred about 18-41% of the time, depending on the year, location, and fungicide.  In early 2009, Pioneer Hi-Bred International published the findings of a multi-year, multi-variety replicated study (Foliar Fungicide Effect on Soybean Yield Pioneer Agronomy Sciences, Crop Insights Vol. 19 No. 1). They found that Headline® and Quadris® produced an economic yield result 51% and 47% of the time, respectively, in the absence of significant disease pressure.  We wrote extensively about the use of fungicides in soybean in a recent KPN article (http://www.uky.edu/Ag/kpn/kpn_09/pn_090714.html).

As with corn, soybean is most likely to experience an economic (or statistically significant) yield response to a fungicide when there is an elevated disease risk.  In the absence of significant, visible disease, the probability of achieving an economic response is about the same as if you had flipped a coin. We still have not seen any data which has convinced us that fungicides should be applied to soybean when the risk of disease is negligible.  Said a different way, it is our opinion that growth efficiency and stress tolerance benefits, while they occur from time to time in soybean treated with Headline® (and other strobilurin fungicides), are currently not predictable and, therefore, should not be the basis for applying a foliar fungicide.

Concluding Points
Strobilurin fungicides, and strobilurin-triazole premixes, are excellent fungicides for disease control in both corn and soybean.  Strobilurin fungicides can also enhance plant growth and crop performance under certain (as of yet undefined) conditions.  However, in validly conducted trials, the growth-promoting and/or stress tolerance benefits, in the absence of significant disease, are not predictable in either corn or soybean.  Furthermore, based on results of validly conducted studies  in low disease circumstances to date, treatment with Headline® and other strobilurins are only likely to produce an economic benefit  about half the time. The other half of the time the grower will lose money. Unfortunately, neither the Headline® supplemental label, nor the section 3 product label, provide farmers with specific instructions or guidance for achieving the growth efficiency/stress tolerance benefits.  The supplemental label also fails to define the realistic limitations for the benefits.  For example, should a producer realistically expect to see a drought tolerance benefit if Headline® is applied to a crop that is already under significant drought stress?  The supplemental label is mute on points of practical application such as this.

Possible negative consequences from the use of fungicides for growth promotion include speeding the development of fungicide resistance, added drying costs because of increased grain moisture content, increased human exposure to strobilurin chemistry, increased environmental contamination, and other concerns. These are just some of the reasons why it is critical to avoid any and all fungicide uses that cannot be well-justified.

Bottom Line
Use strobilurin fungicides based on disease risk.  The higher the risk of diseases like gray leaf spot of corn and frogeye leaf spot of soybean, the better the case for using a fungicide.  Don’t base fungicide spray decisions on unpredictable growth-promoting and stress-tolerance properties.