March 2020 - 13

Anaerobic soil disinfestation curbs high tunnel pathogens
By Dean Peterson
VGN Correspondent
Continuous production of high value
crops in high tunnels - with few rotation
options - can lead to a build-up of
soilborne diseases.
" If you grow tomatoes after tomatoes,
year after year, that will lead to a buildup
of pathogens, " said Sally Miller,
professor in vegetable pathology at Ohio
State University (OSU) and a speaker at
the recent Great Lakes Fruit, Vegetable
and Farm Market EXPO in Grand
Rapids, Michigan.
The build-up of soilborne diseases is
especially troublesome with pathogens
that have many
hosts and if
production
is year-round
with continuous
warm
temperatures.
The list
SALLY MILLER
of problem
soilborne
pathogens that
can plague high
tunnels is long.
Verticillium and fusarium wilts
are both issues with verticillium wilt
especially concerning because it has a
wide range of hosts.
Fungal root rot complexes - black dot
root rot and corky root rot - can also
build up in high tunnels and can occur
separately or together. Black dot appears
in 90% of Ohio's high tunnels and corky
root rot in about half of them.
White mold, rhizoctonia root and
crown rot, and pythium root rot are
other troublesome soilborne diseases.
All of them are long-lived in the soil and
have a broad host range.
Root knot nematode has also become a
problem in high tunnels and can increase
the severity of fungal root rots.
Use of anaerobic soil disinfestation can reduce the amount of root rot pathogens,
increase weed seed mortality and greatly reduce the root knot nematode population.
However, new research at OSU shows
incorporating a carbon source into the
soil of a high tunnel under anaerobic
conditions causes a shift in soil microbial
communities, increased acidity and
production of volatile organic acids that
suppress these soil-borne pathogens.
The process is called anaerobic soil
disinfestation (ASD).
" It utilizes your natural biological
population in the soil, " Miller said. " When
you use ASD, you shift the microbial
population toward those populations that
produce acids and can survive acidic and
anaerobic environments. "
The process is being widely studied
and is being used on a big scale in
California and Florida because of the
loss of methyl bromide.
ASD involves three steps. Growers
incorporate the organic material into the
soil, irrigate the soil to saturation to create
the anaerobic environment and cover
the soil with plastic to keep it anaerobic.
Acetic acid and butyric acid are the main
acids produced by the process.
In OSU's study, ASD reduced the
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VGN | MARCH 2020 | 11
amount of root rot pathogens, increased
weed seed mortality and greatly reduced
the root knot nematode population.
The treatment also achieved good
control of white mold. " The acids chew
up the outside melanized layer of the
sclerotia, " Miller said.
The source of the soil amendment in
ASD is crucial. " When you pick your
carbon source, find something you
can easily get and isn't too expensive, "
Miller said.
The OSU study tested wheat bran and
molasses. The best results came from a
combination of nine tons of wheat bran
and 4.5 tons of molasses per acre.
" First, you spread the wheat bran and
incorporate it with a rototiller or by hand, "
Miller said. The incorporation depth is
six to eight inches. " Molasses is a liquid
and may percolate into the soil better than
wheat bran, " Miller said. You have to dilute
the molasses because it's so thick.
The next step is to water the treated
area to saturation and put the plastic
down carefully. " The plastic edge needs to
be tucked under and covered with soil so
there's a good seal to keep gases in, " Miller
said.
A heavier grade of plastic should be used
and the soil covered for three to five weeks.
The soil can be covered before irrigation if
a drip tape is placed under plastic and used
for watering.
ASD is more effective with warmer
soil temperatures and longer covering
periods. Three weeks should be long
enough to control most pathogens if the
soil temperatures are consistently more
than 85° F.
After irrigation on a coarse-textured
soil, you may have to add more water
after you put down the plastic to keep the
soil saturated.
Check the soil fertility after ASD
treatment. " You're adding a lot of carbon
and the microbes may be binding up your
nitrogen, " Miller said.
Delay planting five to seven days after
removing the plastic to allow time for the
soil to dry and aerate.
ASD has some issues. " ASD reduces the
population of some beneficials, but the
data shows they tend to come back pretty
quickly, " Miller said. There's also the issue of
butyric acid's strong smell so you don't want
to work in the greenhouse until it's aired out.
Most importantly, ASD must be
combined with other management
practices. " This is not a silver bullet, "
Miller said.
Sanitation after harvest is one practice
that must be combined with ASD. Leaving
plants in the soil after harvest leaves hosts
for pathogens.
Weed control is vital. " They harbor
diseases and insect pests, " Miller said.
" Weeds also make the environment more
humid which is a big factor for diseases. "
" We want to prevent the existing
pathogen population from increasing.
We want to reduce the existing pathogen
population and we want to reduce the
seed bank, " Miller said. " That's hard to do
but ASD is an option. " VGN
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March 2020

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