Update On Use Of Strobilurin Fungicides In Corn
DR. DON HERSHMAN
LEXINGTON, KY. PRINCETON, KY.
Strobilurin fungicides are highly effective for
controlling a number of diseases in corn
and other crops. They also are known to
sometimes have certain growth-regulating effects
which often can have positive consequences.
Headline®, Quadris®, and Evito® are
strobilurin-based products. There also are several
pre-mix fungicide products that contain a
strobilurin fungicide as one of the two active ingredients.
Examples include Stratego®, Quilt®,
Priaxor® Xemium®, Avaris®, and Headline AMP®.
Virtually all plant pathologists agree that
these fungicides are valuable for controlling certain
diseases of corn, such as gray leaf spot,
northern leaf blight, and southern rust. However,
there has been some disagreement about
how and when to use strobilurin fungicides in
corn crops. This brief article provides a short
evaluation, and it is based on continuing and
fruitful research and communication among
scientists in both industry and in the public domain.
Based on an assessment of the state of the science
on these products in corn, here are key
points:
1. Disease risk is still the best indicator of
the likelihood of economic benefit from a
fungicide application. See Figure 1 for an
overview of risk factors. If you are considering
using a fungicide on some of a farm’s acreage, it
makes sense to apply them to fields were the
highest risk.
2. Research results seem to converge on
VT/R1 (tasseling/silking) as the best time to
make such an application. Applying the fungicide
at this time generally maximizes the disease
control benefit, and possibly the same is
true for the growth-promoting benefits. There is
some “buzz” happening about the potential yield
benefit of sequential applications of fungicides
in corn in selected situations. These circumstances
may include fields being “pushed” agronomically
(inputs and plant populations, etc.) to
maximize yields. The data supporting such recommendations
are not actually public, to our
knowledge. Public research thus far doesn’t
show a meaningful benefit to the two-application
approach, so for now, we question the application
of two or more fungicide sprays to corn
in our region. We’ll keep an open mind on this
topic and watch for a body of solid evidence that
justifies the two-application approach. In the
meantime, we encourage producers to exercise
skepticism about sequential applications. Try
leaving a couple untreated strips when making
the first application. The “sequential application”
strategy says that the yield on strips
treated only at VT/R1 will be significantly lower
than where two applications were made. See if it
turns out that way on your farm.
3. There is some concern about the risk of
fungicide resistance. The strobilurin fungicides
are widely known by scientists to generally
have a moderate to high risk for
development of resistance. Consider frogeye leaf
spot in soybeans. Dr. Carl Bradley of the University
of Illinois has reported strobilurin-resistant
strains of Cercospora sojina (the fungus
that causes soybean frogeye leaf spot) in eight
states, including Kentucky. Follow-up research
by Don Hershman has found that over half of
the Kentucky isolates of C. sojina tested thus far
were resistant to strobilurins. Will resistance
develop in important corn pathogens? In our
opinion, it seems likely, although probably not
as readily as in important diseases of other
crops. The three corn diseases for which fungicides
can be of particular value in Kentucky are
gray leaf spot, northern leaf blight, and southern
rust. The good news for producers is that
the fungi that cause these diseases all possess
a “genetic quirk” that interferes with the development
of mutants with high levels of resistance.
If a fungus possess that particular genetic
quirk and a mutation occurs that can give it a
high level of resistance, that mutation is actually
lethal. Yes, the fungus “commits suicide” by
mutating to high resistance to the strobilurin
fungicides. Pretty slick! But while that is the
good news, there still is some risk from repeated
use of these fungicides against foliar diseases of
corn, for the following reasons.
a. We don’t know if that genetic quirk exists
in all field populations of these fungi.
b. This genetic quirk does not prevent the development
of moderate levels of resistance to
strobilurins, only high levels of resistance.
Crop response under low disease
A key issue that still remains poorly resolved
is crop response to strobilurin fungicides when
disease pressure is minimal. There is no question
that application of a strobilurin fungicide
can sometimes result in a yield increase in corn
under low disease pressure. On some occasions,
these yield increases can be as large as
20+ bushels. The problem is that these increases
do not seem consistent or predictable.
A legitimate question is, are small-plot trials
able to adequately measure the physiological
benefits of strobilurin fungicides? It seems like
a simple question, but answering this has
proven extremely challenging. Research to date
suggests that, in fact, there are some limitations
to small-plot designs. This topic can get complex
really quickly, so we’ll just make a few
points:
• Variability may be increased in some smallplot
designs. Having results from numerous trials
should help to overcome this limitation, but
not completely.
• The average “yield bump” obtained from a
strobilurin fungicide in a small-plot design may
be slightly off, but in our trials, this difference
appears to be small to even non-existent. Some
industry trials suggest a larger difference exists
than in our trials.
We’ll keep an open mind about this issue of
small-plot trials vs. on-farm strips, although we
suspect that this question will not be resolved in
our lifetimes. We wish research would continue
to address this question of how to best evaluate
fungicides. That way, we would be able to provide
producers with the best scientific assessments
possible. However, such studies are
difficult to conduct, and funding for them is
nonexistent. Therefore, we expect this important
line of applied research to be at a standstill.
Summary
Research still seems to indicate that the best
use of fungicides in corn is as single application
at VT/R1 in fields with significant disease risk.
But beyond this, weighing the costs and benefits
of fungicide use in corn still is complicated,
even after years of field research.
Acknowledgments
Thanks to Carl Bradley (University of Illinois)
for reviewing a previous draft of this article. Δ
DR. PAUL VINCELLI: Extension Plant Pathologist,
University of Kentucky
DR. DON HERSHMAN: Extension Plant Pathologist,
University of Kentucky