Know Your Nematodes
Soil Test, Scout To Curb Nematode Damage In Crops
LELAND, MISS.
Nematodes are the most abundant multicelled
animal in the world; they are found
wherever there is water. While there are
many different types, Dr. Gabe Sciumbato, Mississippi
State research plant pathologist, recently
discussed the kind that parasitize plants.
“There are about 24 species,” he began, “but
the main three I am talking about are on soybeans,
and those are the root knot, the reniform
and the cyst nematodes. The thing about plant
parasitic nematodes is they are what they call
obligate parasites. They have to
live on a living organism. They
can’t live on leaves or anything
else, it has to be alive. If they
don’t have a living organism to
live on, they die.”
The root knot nematode can
cause as much as a 30 percent
to 40 percent yield loss on soybeans.
Only an extremely bad
cyst nematode infestation can
cause more damage. Root knot
populations are very erratic in
the fields. There is resistance to
the root knot nematode in some
varieties.
“The reniform nematode is a
subtropical and tropical nematode
and it probably causes a
lot more damage in the world
than the root knot,” Sciumbato
added. “It has moved into the
south from the tropics. It’s attracted
to 160 different species
of plants and it’s very hard to
tell that your losing yield from
these nematodes. The reniform
shows no external symptoms
other than the plant doesn’t
yield as good, it may not be
quite as tall; but you can’t tell
by looking at the plants that
they’re infested with nematodes.
“If a field is infested with the
reniform nematode, everywhere
there is a root, there are reniform nematodes,”
he said. “Researchers got a backhoe and dug
down 10 or 15 feet and if there was a root there,
these nematodes were attached to it. They can
build up in tremendous numbers if you grow a
susceptible variety year after year. The threshold,
in other words the level of nematodes that
can cause economic loss given the right environmental
conditions for reniform, is 1,000 to
5,000; but we have seen in a lot of cotton fields,
20,000 to 40,000 nematodes per pint of soil.
That’s 40,000 nematodes in one little pint of
soil. They build up through the years and the
amount of damage and yield loss you’re going
to have is dependent upon how many are present
at the beginning of the year – unless you do
something about it.”
Corn is resistant to the reniform nematode,
and that is one good way of knocking the numbers
back. Rotating to corn after soybeans or
cotton can knock them back. However, if susceptible
soybeans or cotton are grown for one
year reniform nematode build right back up to
where they were before. Also, corn is susceptible
to the root knot nematode.
“With the loss of Temik, we’ve lost a real good
tool for managing nematodes,” Sciumbato said.
“And in Mississippi there has been a reduction
in cotton acreage, and our soybean acres have
moved into lands where cotton had been grown
on lighter soil types. Nematodes like light or
mixed soils. They don’t cause much yield loss
on the heavy clays where we used to grow soybeans.
They do a lot of damage on the lighter
soils. The root knot is the worst on the real light
soil but the reniform likes the medium textured
soil.”
The cyst nematode probably causes the most
damage throughout the United States, especially
in the north.
“We had a big problem with cyst nematode in
the 1970s when we grew a lot of soybeans on
lighter soils,” he recalled. “Then the price of soybeans
went down and we moved into the heavy
soils only and a fungus came through and killed
a lot of the nematodes, so we haven’t had much
problem with the cyst nematodes lately; but a
lot of soybeans are being grown on lighter soils
now and it takes several years for the cyst to
build up to real high proportions where it can
cause severe yield losses. We’re having a lot
more races of the fungus now so resistance is
harder to come by, so I look for the cyst nematode
to become much more of a problem. We
just don’t have a whole lot of tools to control
these nematodes other than rotation and resistant
varieties.”
There are some nematode seed treatments
farmers can use, but with the reniform there is
no resistance in commercial cotton varieties
and very little resistance in high yielding soybean
varieties. So many farmers who have been
growing cotton and now growing soybeans not
realizing that they are building up their nematode
numbers.
“So I look for more and more problems from
this and it’s going to take a lot of attention on
the farmer’s part to sample the soil, see what is
in the soil, and do what it takes to keep these
numbers down.”
Root knot and cyst nematodes were always
here but the reniform only showed up in
Louisiana in the 1950s and 1960s. Since then,
it has moved its way up. It can travel around a
lot more on implements. It can take desiccation
and overwintering more than the root knot or
the cyst, and it moves around a lot. It has become
the number one problem in the Mississippi
Delta. Farmers must soil sample and pay
close attention to the crop to manage these
things.
That’s a big part of Sciumbato’s take-home
message:
“If you’re growing soybeans on a lighter soil
type, not buckshot, then you need to take soil
samples, see what you have, and then decide
on a course of action. Be aware of what you’re
planting. There should be some reniform resistant
cotton varieties in a few years, there are
already some resistant soybean varieties, if you
can find them.
“You can rotate to corn. There are all kinds of
control methods that you can use and they
need to be used appropriately in the right way.
Seed treatment nematicides are not recommended
if you have extremely high numbers of
reniform nematodes. So you just really have to
be aware of what’s going on in your field and
take appropriate action.” Δ
BETTY VALLE GEGG-NAEGER: Senior Staff
Writer, MidAmerica Farmer Grower
Dr. Gabe Sciumbato, Mississippi State
Research Plant Pathologist,
recently discussed some
of the 24 species of nematodes,
the kind that parasitize plants.
Photo by John LaRose, Jr.