April 2021 - 18

PLANT & SOIL HEALTH
Pursue best practices for fumigation, building soil microbes
By Zeke Jennings
VGN Correspondent
Fumigation can be a necessary means
to get soilborne pathogens under
control in potato fields. Chemical
fumigants reduce the bad in the soil,
but also reduce the good, so other soil
health practices are essential following
fumigation.
A panel of three industry experts
came together for a discussion on the
subject during Potato Expo 2021. They
were longtime AMVAC Technical Sales
Rep Ralph Frederick, University of
Minnesota Department of Soil, Water
and Climate Head Carl Rosen and
University of Idaho Department of Plant
Sciences Professor Mike Thornton. The
panel was hosted by Washington state
potato grower Grant Morris.
Let's start by addressing something
of a myth: Do fumigants remove all
good microbes from the soil?
Mike Thornton: Fumigants do
not completely take all of the good
microbes out of the soil. Even the most
extreme example I can think of, which
is something like methyl bromide
under a plastic tarp, we know it does
not kill all of the beneficial organisms.
Different species rebound at different
rates, so it is a really complex reaction
to fumigation.
Carl Rosen: We can say for sure it
does reduce microbial activity, but it
doesn't go to zero. The soil is not sterile.
What happens over time is the microbes
within that soil tend to rebound toward
the end of the season. ... It does go
down, but it does come back again.
How does fumigation fit into a
program that's using biofumigants?
For example, a mustard that we're
chopping and working into the
ground.
Rosen: If you have a really severe
case of a pathogen, like a verticillium or
nematodes, you probably want to use a
conventional fumigation to knock those
microbes down. Then, you can come
back in a year or two with biofumigants
to help keep populations down. So, they
can work hand-in-hand. At the same
time, when you use those biofumigants,
you'll be adding some benefits to
your soil with some carbon and other
biomass that will help your soil.
Ralph Frederick: When (pathogen)
levels are really high, a conventional soil
fumigation is probably best for growers
to start out with. Then, depending on
crop rotations and variety of potato they
grow - different varieties can build up
different pathogens at different
rates - biofumigants can certainly fit into
their direct, well-rounded program. A
challenge of biofumigants, particularly in
the Midwest, is to grow enough biomass
to get the benefits that you need.
Thornton: It's important to keep in
mind that not all green manures control
18 | VegetableGrowersNews.com
Do certain biofumigants work
better with certain conventional
fumigants?
Frederick: They each add different
levels of isothiocyanates (ITC) into the
soil, so you have to keep that in mind
when choosing your product in fitting
them into your rotation.
Rosen: There is still research that
needs to be done in this area. I would
say there are certain biofumigants
that work better on verticillium than
nematodes, but some of that we're still
trying to work on and figure out. In the
meantime, using biofumigants is still
beneficial just by adding more carbon
to your soil and improving microbial
activity.
How do we best fit fumigation into a
soil health program?
Frederick: It gets you to a starting
point by bringing down the detrimental
pathogens. Also your beneficial
(microbes) come down, but you can
build those back up over time. I think
fumigation is a great first step.
Thornton: Pathogens are one aspect
of soil health, and fumigants do play a
role in reducing pests in the soil, but
from a soil health standpoint, fumigants
are not a substitute for a good rotation.
We still need to use high-residue crops,
green manures and use cover crops
when we can to build organic matter
and carbon in the soil, which help to
suppress those diseases.
Rosen: Something that tends to
be forgotten is that you're not only
affecting the pathogen population,
but you're improving the root health
of your crop. That allows potatoes to
explore more soil and utilize nutrients
more efficiently (if you have soilborne
pathogens affecting your root growth).
How do we best maintain our soil
health after fumigation?
Rosen: Mike already talked about
using high-residue (rotational) crops
and cover crops whenever possible,
CARL ROSEN
all pests. There are differences between
mustards and radishes, for example,
and even differences in cultivars. One
cultivar might be effective on a certain
species of nematode, but another
cultivar is not.
MIKE THORNTON
but another thing is tilling as little as
possible not only during your potato
year, but your rotation years too. Green
manures are great, but also any other
type of compost that you have access to.
Frederick: Fumigation is just a tool in
the toolbox, it doesn't replace rotational
crops and green manures that put that
carbon and organic matter back into
the soil. That's really the way to build up
that soil health.
Is there one soil health tip or
practice that really jumps out to you
in relation to fumigation?
Rosen: You just have to remember
that when you fumigate, you are
bringing down the (microbes) in your
soil, so you have to take those steps
along the way (that we've talked about
above) to rebuild those.
Frederick: Every field is different.
So, if you have potatoes in a field for
a long time, you're going to have a
different level of pathogens in that field
than if it's a virgin field or field where
you've grown very few potatoes before.
Fumigation lets you kind of get back to
that even point, and allows you to build
your soil back up with a good rotation
and by putting in as much organic
matter as possible.
Thornton: To me, fumigation is
not the key piece of the soil health
discussion. Whether you fumigate or
not, it doesn't change the need for a
good rotation, the need to pay attention
to your tillage or the need for green
manure or cover crops when you can.
If someone is using a lot of
biopesticides or amendments and
adding healthy microbes into the
ground all year long, is there still a
need to fumigate?
Frederick: It really depends on where
your bad pathogen level is at. Fumigation
is an expensive treatment, so people don't
do it haphazardly. If they're doing it, there
is an assumption that there is something
wrong with that field. ... Even if you're
adding all the healthy microbes, if you
reach a certain level (of a pathogen), you
may still need to fumigate to bring the
detrimental pathogen levels down to a
manageable level.
Thornton: We've done work with
biological fungicides and biofumigants
RALPH FREDERICK
on where they work and maybe where
they don't. In general, the efficacy is not
what you would see with a traditional
chemical-based fumigant. If you're in a
situation where you have a susceptible
variety and a high pest population, they
generally don't do an adequate job. If
you have a resistant variety and you're
growing on soil with low populations of
pests, then you might want to consider
a biological as an alternative. It's really a
case-by-case situation.
Rosen: You definitely want to have
your soil tested to make sure it's at
a level that warrants fumigation.
Secondly, do adjust by the variety you're
growing.
Is there one or two things you can
think of to recommend to growers on
fumigation for this season?
Rosen: For this year's crop, I would
assume most growers have fumigated
already, so really just follow the best
practices for producing that crop.
Make sure that you apply appropriate
nutrients, for example, and make sure
your foliar diseases are controlled
so you can get the benefits of those
reduced soil pathogens.
Frederick: You've set yourself up for
a good crop with soil fumigation. Now,
you want to make sure you inspect your
seed and it's clean. Make sure you don't
have black scurf, or at least limited
black scurf, so you're not bringing
rhizoctonia back into the field. You also
don't want to do any deep tillage. You
don't want to pull up untreated soil into
your treated soil. Make sure you bring
in clean equipment, too.
Thornton: From a general fumigant
standpoint, we tend to focus a little too
much on the right rate application and
forget about all the other factors that
determine how well they work. I'd like
to see us take more of an approach like
they do with nutrients and the four Rs.
Rate application and cost is a key, but
so is the right time and making sure
the soil is the right temperature and the
pest is active, the right place in knowing
where the pest is your soil. Finally, the
right conditions - we talk a lot about
soil moisture and till in fumigant
application, and if those aren't right,
you can put on a lot of fumigant and
not get the efficacy. VGN
http://www.VegetableGrowersNews.com
April 2021

Table of Contents for the Digital Edition of April 2021
