Some Principles Of Fungicide Resistance VII: Adaptability Of Plant Pathogens
This is seventh in the series
LEXINGTON, KY.
I have learned never to underestimate
plant
pathogens. One of the most
important principles of fungicide
resistance is that microorganisms,
such as plant
pathogens, are remarkably
adaptable.
From a practical standpoint,
what this means is that we cannot assume that
resistance will never develop to the fungicides
we use for disease control. This is especially the
case for the many new products with very specific
modes of action.
Here is an example. In a previous article in
this series, I wrote about how fungicide resistance
can be like a coat of armor (Figure 1), protecting
the fungus when fungicide is sprayed,
but weighing it down in the absence of fungicide.
In such a case, we say that there is a “fitness
cost” for resistance to that fungicide.
So if there is a heavy cost to resistance to a
pesticide, what might microorganisms do?
Sometimes they genetically fix that problem, in
two steps:
• First, they develop the resistance to the toxin
(the heavy armor);
• Then, over several generations, they evolve a
progressively lighter and lighter armor, to the
point where they still carry the protective armor,
but it is no longer a burden.
This process, called “compensatory mutation”,
has been documented in bacteria, though to my
knowledge, not in fungi. But honestly, I know of
no reason why it shouldn’t happen in fungi.
After all, if the armor is heavy, basic evolutionary
biology suggests that strains carrying lighter
armor will provide a competitive advantage.
Another example: As discussed in the first article
in this series, mutation (Figure 2) is a driving
force behind the development of fungicide
resistance. It turns out that mutation rates
vary, depending on the environment. It is especially
interesting that environmental stress can
actually trigger higher mutation rates in some
microorganisms. In other words, under a stressful
environment, the genetic machinery of microorganisms
may generate more variants than
normal. This is highly adaptive, since some of
the new variants might be “just right” for the
new environmental conditions. In fact, in bacteria,
researchers have shown that antibiotics
can actually increase the rate of mutation. It is
interesting to wonder whether this may happen
in response to pesticide application.
Commercial fungicides are recent inventions,
so how is it that fungi even have genes for resistance
to these materials? The fact is that microorganisms
typically use genes that have
evolved for other purposes. For example, fungi
sometimes resist a fungicide by simply pumping
it out of the cell using a molecular “efflux
pump” (look at the top of Figure 3). Efflux
pumps have existed for millions of years, so
they didn’t evolve specifically to resist modern
fungicides. They evolved to pump out naturally
occurring toxins. However, they often work well
against our modern pesticides. In fact, they
usually work against a variety of unrelated toxins,
so they are said to give the microorganism
“multi-drug resistance”. Thus, fungi may sometimes
be “pre-adapted” to resist our modern
fungicides, because they possess an efflux
pump. We simply select these resistant strains
by applying fungicides.
Bottom line
Fungi and other microorganisms are remarkably
adaptable. Based on this principle, a conservative
assumption is that fungi will find ways
to adapt to the fungicides we use for disease
control, especially against the many new products
with very specific biochemical modes of action.
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/)
Figure 2. The double helix of DNA, showing a few letters of the
genetic code. A change in the sequence of letters is an example of
a mutation. Image from
http://www.ninds.nih.gov/disorders/brain_basics/genes_at_w
ork.htm.
Figure 3. Diverse ways that amicrobemay resist a poison. Image
from http://www.asu.edu/courses/css335/ar.htm