Fargo, North Dakota
April 23, 2009
Less than ideal planting
conditions (floods, waterlogged soils, cold temperatures) may
challenge the establishment of crops this summer. One common
concern plant pathologists have in these conditions is the loss
of stand and yield due to seedling blights and root rots.
Poorly drained and wet or cold soils will slow crop germination
and favors soil- borne pathogen activity.
"Examples of fungi that can cause seed and root rot under wet,
cold conditions include various Pythium species that attack many
crops, the Phytophthora root rot fungus of soybeans and the
downy mildew fungus that attacks sunflowers," says Marcia
McMullen, North Dakota State
University Extension Service plant pathologist.
"If the soils become warmer, but are still wet, conditions favor
infection by a number of other soil-borne fungi, such as the
common root rot fungus on small grains and the Fusarium and
Rhizoctonia root rot fungi of multiple crops. Re- ccurring wet
conditions following germination and emergence also favor root
rot development."
Even if soil conditions improve later in the season, early
infections still may rob yield.
"Sometimes the greatest yield loss from root rot occurs in years
that begin wet and become dry," says Sam Markell, NDSU Extension
Service plant pathologist.
"When water is abundant (wet soils), even an infected root
system may be able to extract enough moisture for the plant.
However, when the soil becomes dry, a diseased root probably
will not have the ability to extract enough moisture to reach
maximum yield potential. Additionally, Fusarium and Rhizoctonia
may continue to develop even when soil begins to dry, which will
exacerbate the problem."
Fortunately, many broad-spectrum seed treatment fungicides are
available to protect against soil-borne fungi, as well as
protect against seed-borne diseases. Many contain mefenoxam or
metalaxyl fungicides that protect the seedling against water
mold fungi, such a Pythium or Phytophthora. They also contain a
fungicide, such as a triazole or strobilurin, that acts against
most fungi other than the water molds.
"A product that contains a combination of chemicals with
different targets (broad-spectrum seed treatment) generally is
recommended under these disease- favoring conditions," Markell
says. "Otherwise, a mix of single-acting products may be needed.
Some commercial products contain two or more of these
fungicides, while others may contain only one."
Some of the broadest spectrum fungicide seed treatments
registered for wheat and barley are Dividend (difenoconazole +
mefenoxam), Cruiser Maxx (difenconazole + mefenoxam +
thiamethoxam insecticide), Raxil or Dyna Shield (tebuconazole +
metalaxyl), Raxil MD Extra --W (tebuconazole + metalaxyl +
imidacloprid
insecticide) and Charter PB (Triticonazole + thiram). All of
these products protect against multiple root rots, seedling
blights and loose smut, plus several have protection against
soil-borne insects, such as wireworm.
"In general, the response to the use of these seed treatments
has resulted in stand improvements across a wide set of
environmental conditions and wheat yield increases of 2 to 6
bushels on average," McMullen says. "Seed treatment costs vary,
but range up to $2 per bushel depending on the manufacturer and
disease spectrum of the product."
Field history, such as continuous cropping, short rotations and
a history of root rot, increase the risk of root rot problems in
broad-leaf crops, many of which have similar root pathogens
attacking across multiple crops. Poorly drained or cold, wet
soils also delay crop emergence and favor growth and infection
by these root rot fungi. When choosing a fungicide seed
treatment for broad leaf crops, it generally is best to apply a
broad-spectrum product with activity against Pythium and
Phytophthora (mefenoxam or metylaxyl) and Fusarium and
Rhizoctonia (for example, azoxystrobin, trifloxystrobin and
fludioxonil), or a mixture of single- product ingredients may be
needed.
In a two-year (2003 and 2004), 14-location soybean fungicide
seed treatment study in North Dakota, the average yield increase
was 2 to 4 bushels per acre, depending on treatment. At $10 per
bushel soybeans, that would translate into a $20- to
$40-per-acre return on investment, minus the cost of the
treatment. However, that data was averaged over two very
different environments. In a dry spring (2003), no yield
increase was observed in any of the six locations tested.
In a wet and cool spring (2004), a statistically significant
yield increase was observed in four of the eight locations
studied. Additionally, statistically significant increases in
stand and reductions in disease were observed in two additional
locations. Thus, the potential return on a seed treatment may be
even higher in a cool and wet spring.
Most seed treatment products are registered for on-the-farm use,
either for a drill box application or to be applied in a mist or
slurry with an auger treater. For information on current seed
treatment products registered on all crops in North Dakota,
check out the "2009 North Dakota Field Crop Fungicide Guide"
(Extension publication PP-622) available at
http://www.ag.ndsu.nodak.edu/extplantpath/.
Always read the label carefully and follow the label directions
for application procedures, rates and specific diseases
controlled. |
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