Some Principles Of Fungicide Resistance V: Ecological Fitness
This article is the fifth in the series.
LEXINGTON, KY.
Previous articles in this
series have presented
basic concepts about
how fungicide resistance develops
in populations of infectious
fungi. This article
presents a more advanced
concept, but one that is key to
understanding fungicide resistance.
Ecologists use the term “fitness” to describe
the overall ability of an organism to thrive and
reproduce in a given environment. Many qualities
contribute to ecological fitness. An obvious
example is fungicide resistance. In a crop field
where a fungicide is being used, if a spore has
genetic resistance to that fungicide, it is more
“fit” than a spore that doesn’t. Think of fungicide
resistance like a coat of armor, protecting
the fungus from the fungicide (Figure 1).
Let’s take the “armor” metaphor a little further.
On the battlefield, having a coat of armor
is beneficial. However, in daily life, having to
wear a coat of armor would get tiresome very
fast. Sometimes, this is how it is with fungicide
resistance. The genetic resistance to fungicides
helps protect the fungus for as long as the fungicide
is being used. However, if the producer
stops using the fungicide – or switches to a fungicide
in another FRAC group – the genetic resistance
to fungicides actually may be a burden,
like an unnecessary coat of armor.
Here are some real-world examples:
• When resistance develops to strobilurin
fungicides (azoxystrobin, trifloxystrobin, pyraclostrobin,
and other FRAC Code 11 fungicides),
it commonly confers very little to no fitness cost.
It is as if the armor were weightless.
• Resistance to the many triazoles and related
fungicides (FRAC Code 3) often results in a
modest fitness cost, like wearing light-weight
armor.
• Resistance to dicarboximides (FRAC Code 2)
often comes at a significant fitness cost to the
fungus, as if the armor it was carrying was very
heavy.
Although you may have never heard of ecological
fitness before, it really can work to a producer’s
advantage, or disadvantage. Imagine
that a fungicide-resistant spore occurs on your
farm. Here is the range of possibilities:
• If you are lucky, that genetic resistance to
fungicides may have a “fitness cost”, (=heavy
armor). If so, that fungal strain may “limp
along” and cause disease on your farm as long
as you continue to use fungicides in that FRAC
group. However, if you stop using those fungicides,
the resistant strain will commonly begin
to die out, and it may eventually return to very
low levels on your farm. If there is a substantial
fitness cost to fungicide resistance, you can
commonly go back to using the fungicide, at
least for awhile, until resistant strains build up
again.
• If you are unlucky, the resistant strain will
have absolutely no fitness cost, as if the coat of
armor weighed nothing at all. What this usually
means is, you are stuck with resistance indefinitely.
Even if you stop using fungicides in that
particular FRAC group, the resistant strain will
persist for a long time.
On a given farm, either of these two extremes
may occur, as can outcomes intermediate between
these extremes.
So here are some practical questions that follow
from this concept of ecological fitness:
•1. When fungicide resistance occurs, how fit
are the resistant strains? It is a key question,
but it takes quite a bit of research to answer it
for any given case of resistance. It is complicated
by the fact that each new fungal strain,
like people, is a unique individual, and we will
only know how well-adapted a strain is by
watching how it does in nature. However, one
thing is for sure: the occurrence of resistance
does not necessarily pose a threat to a farming
operation, depending on how fit the resistant
strain is.
2. How can we manipulate the ecological fitness
of pesticide-resistant microbes? Great
question. But we can’t. We have no influence on
whether or not the fungal spores in a particular
field carry a heavy coat of armor or a weightless
one. We can only reduce the risk that the coat of
armor will arise on its own (through mutation).
You can only hope that, once it arises, the coat
of armor is heavy. In ecological terms, we can
only reduce the chance that a fit mutant will
occur in our fields, but we cannot influence
whether there is a fitness cost to that resistance.
3. How can we reduce the chance of a fit mutant
occurring in our fields? The only way to reduce
the risk of the fit mutant is by reducing
disease activity on the farm. See the third article
in this series for more on this topic, but basically,
it means using resistance varieties and
cultural practices to reduce disease pressure.
The lower the disease pressure, the lower the
chance that a fit mutant will spontaneously
occur.
Bottom line:
Fungicide resistance is like a coat of armor,
protecting the fungus from the fungicide. In
some cases, the coat of armor is heavy, becoming
a burden to the fungus in the absence of
fungicide. This is referred to as a “fitness cost”
to the fungicide resistance. If resistant strains
in your field carry a fitness cost, sometimes it is
possible to still use that fungicide selectively,
because the resistant strain is may die out during
periods when that fungicide is not applied.
In contrast, if there is no fitness cost to resistance,
resistant strains will likely stick around
for a long time. Δ
DR. PAUL VINCELLI: Extension Professor and
Provost’s Distinguished Service Professor, University
of Kentucky
Figure 1. Imagine fungicide resistance being like a coat of armor,
protecting the spore from the chemical poison. (Image of "ring
armor", retrieved 23 June 2013, from http://etc.usf.edu/clipart/)