28 Landscape and Irrigation April 2014 www.landscapeirrigation.com
Although
there are many different modes of action available,
many of the most popular fungicide products for turf contain in-
gredients with the same mode of action. This brings up the potential
problem of fungicide resistance. Fungi are highly diverse, and re-
peatedly spraying the same mode of action selects for any resistant
individuals that happen to be present in a given population. If they
are, then they multiply while susceptible ones are killed and soon
the majority of the fungal population is resistant, and the fungicide
stops working to prevent disease.
Modern fungicides tend to have very specific sites of action in fungi.
For example, many target just one enzyme in a fungus, binding to it
and making it no longer able to function. This is good, since it means
that the fungicides are very specific and less likely to cause harm to
non-target organisms. But it is also bad from a resistance standpoint,
because it means that all that has to happen for a fungus to become re-
sistant is a small change in that one enzyme such that the fungicide
can no longer inactivate it. This happens in nature, and often just one
or two mutations are enough to make a fungus resistant.
Because of the potential for many turfgrass diseases to become re-
sistant to fungicides, managers have been advised for years to rotate
modes of action or to tank mix more than one mode of action at a
time in a given application. The international Fungicide Resistance
Action Committee (FRAC, www.frac.info) maintains a listing of cur-
rently
registered fungicide active ingredients and their modes of action.
They are sorted into groups of individual ingredients sharing the same
mode of action and each group is assigned a unique code.
Recently, manufacturers have begun placing the FRAC mode of
action group codes on their product labels. This is a tremendous help
to the turf manager trying to manage resistance, as now it is immedi-
ately obvious which products contain ingredients that have the same
mode of action. Now it is possible to tell at a glance whether rotating
to a given product will actually mean switching modes of action.
It is important to remember, though, that even though resistance
has been documented in many turfgrass diseases, not every failed fun-
gicide application is due to resistance. It is still more common to see
fungicides fail due to improper calibration, reading labels incorrectly,
not using enough spray volume and/or the wrong nozzles, and plain
old misdiagnosis of the disease. Nevertheless, if you suspect resistance,
it is a good idea to contact your local Extension agent or plant pathol-
ogy lab. They will be able to assist you in identifying possible problems
with your fungicide application and, if needed, can collect samples and
screen them for resistance.
Dr. Dave Han is an associate professor of Crop, Soil and Environmental
Sciences at Auburn University, and an Extension Specialist at the Alabama
Cooperative Extension System.
Landscape and Turf Maintenance
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